diff --git a/internal/api/adapter.go b/internal/api/adapter.go
index e33976e..e78fcc6 100644
--- a/internal/api/adapter.go
+++ b/internal/api/adapter.go
@@ -104,11 +104,7 @@ func (a *HandlerAdapter) DeleteV1JupyterServicesServiceId(ctx echo.Context, serv
 		})
 	}
 	// TODO: Implement when StopServiceHTTP is available
-	return ctx.JSON(501, map[string]any{
-		"error":   "Not implemented",
-		"code":    "NOT_IMPLEMENTED",
-		"message": "Jupyter service stop not yet implemented via REST API",
-	})
+	return toHTTPHandler(a.jupyterHandler.StopServiceHTTP)(ctx)
 }
 
 // GetV1Queue returns queue status
diff --git a/internal/api/factory.go b/internal/api/factory.go
index e9313e9..c65c0b7 100644
--- a/internal/api/factory.go
+++ b/internal/api/factory.go
@@ -44,6 +44,11 @@ func (s *Server) initializeComponents() error {
 		return err
 	}
 
+	// Initialize default lab group (if configured)
+	if err := s.initDefaultLabGroup(); err != nil {
+		return err
+	}
+
 	// Initialize security
 	s.initSecurity()
 
@@ -185,6 +190,31 @@ func (s *Server) initDatabaseSchema() error {
 	return nil
 }
 
+// initDefaultLabGroup creates the auto-provisioned default lab group if configured.
+// Reads DEFAULT_LAB_GROUP env var and creates the group if it doesn't exist.
+// If DEFAULT_LAB_GROUP is not set, this is a silent no-op (intentional - groups are optional).
+func (s *Server) initDefaultLabGroup() error {
+	if s.db == nil {
+		return nil
+	}
+
+	groupName := os.Getenv("DEFAULT_LAB_GROUP")
+	if groupName == "" {
+		return nil // No default lab group configured
+	}
+
+	groupID, err := s.db.GetOrCreateDefaultLabGroup("system")
+	if err != nil {
+		s.logger.Error("failed to initialize default lab group", "error", err)
+		return err
+	}
+
+	if groupID != "" {
+		s.logger.Info("default lab group initialized", "group", groupName, "id", groupID)
+	}
+	return nil
+}
+
 // initSecurity initializes security middleware
 func (s *Server) initSecurity() {
 	authConfig := s.config.BuildAuthConfig()
diff --git a/internal/api/groups/handlers.go b/internal/api/groups/handlers.go
new file mode 100644
index 0000000..d6a7eac
--- /dev/null
+++ b/internal/api/groups/handlers.go
@@ -0,0 +1,692 @@
+// Package groups provides HTTP handlers for lab group management
+package groups
+
+import (
+	"encoding/binary"
+	"encoding/json"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/gorilla/websocket"
+	"github.com/jfraeys/fetch_ml/internal/auth"
+	"github.com/jfraeys/fetch_ml/internal/logging"
+	"github.com/jfraeys/fetch_ml/internal/storage"
+)
+
+// Handler provides group management HTTP handlers
+type Handler struct {
+	db     *storage.DB
+	logger *logging.Logger
+}
+
+// NewHandler creates a new groups handler
+func NewHandler(db *storage.DB, logger *logging.Logger) *Handler {
+	return &Handler{
+		db:     db,
+		logger: logger,
+	}
+}
+
+// CreateGroupRequest represents a request to create a new group
+type CreateGroupRequest struct {
+	Name        string `json:"name"`
+	Description string `json:"description,omitempty"`
+}
+
+// CreateGroup handles POST /api/groups
+func (h *Handler) CreateGroup(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	// Only admins can create groups
+	if !user.Admin {
+		http.Error(w, "forbidden: admin required", http.StatusForbidden)
+		return
+	}
+
+	var req CreateGroupRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		http.Error(w, "invalid request body", http.StatusBadRequest)
+		return
+	}
+
+	if req.Name == "" {
+		http.Error(w, "name is required", http.StatusBadRequest)
+		return
+	}
+
+	group, err := h.db.CreateGroup(req.Name, req.Description, user.Name)
+	if err != nil {
+		h.logger.Error("failed to create group", "error", err)
+		http.Error(w, "failed to create group", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("group created", "group_id", group.ID, "name", group.Name, "created_by", user.Name)
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusCreated)
+	if err := json.NewEncoder(w).Encode(group); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// ListGroups handles GET /api/groups
+func (h *Handler) ListGroups(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	groups, err := h.db.ListGroupsForUser(user.Name)
+	if err != nil {
+		h.logger.Error("failed to list groups", "error", err)
+		http.Error(w, "failed to list groups", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]interface{}{
+		"groups": groups,
+		"count":  len(groups),
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// CreateInvitation handles POST /api/groups/{id}/invitations
+func (h *Handler) CreateInvitation(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	groupID := r.PathValue("id")
+	if groupID == "" {
+		http.Error(w, "group id required", http.StatusBadRequest)
+		return
+	}
+
+	// Check if user is group admin
+	isAdmin, err := h.db.IsGroupAdmin(user.Name, groupID)
+	if err != nil || !isAdmin {
+		http.Error(w, "forbidden: group admin required", http.StatusForbidden)
+		return
+	}
+
+	var req struct {
+		InvitedUserID string `json:"invited_user_id"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		http.Error(w, "invalid request body", http.StatusBadRequest)
+		return
+	}
+
+	if req.InvitedUserID == "" {
+		http.Error(w, "invited_user_id is required", http.StatusBadRequest)
+		return
+	}
+
+	invitation, err := h.db.CreateGroupInvitation(groupID, req.InvitedUserID, user.Name)
+	if err != nil {
+		h.logger.Error("failed to create invitation", "error", err)
+		http.Error(w, "failed to create invitation", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("invitation created", "invitation_id", invitation.ID, "group_id", groupID, "invited_user", req.InvitedUserID)
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusCreated)
+	if err := json.NewEncoder(w).Encode(invitation); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// ListInvitations handles GET /api/invitations
+func (h *Handler) ListInvitations(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	invitations, err := h.db.ListPendingInvitationsForUser(user.Name)
+	if err != nil {
+		h.logger.Error("failed to list invitations", "error", err)
+		http.Error(w, "failed to list invitations", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]interface{}{
+		"invitations": invitations,
+		"count":       len(invitations),
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// AcceptInvitation handles POST /api/invitations/{id}/accept
+func (h *Handler) AcceptInvitation(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	invitationID := r.PathValue("id")
+	if invitationID == "" {
+		http.Error(w, "invitation id required", http.StatusBadRequest)
+		return
+	}
+
+	// Verify invitation belongs to this user and is pending
+	invitation, err := h.db.GetInvitation(invitationID)
+	if err != nil {
+		http.Error(w, "invitation not found", http.StatusNotFound)
+		return
+	}
+
+	if invitation.InvitedUserID != user.Name {
+		http.Error(w, "forbidden", http.StatusForbidden)
+		return
+	}
+
+	if invitation.Status != "pending" {
+		http.Error(w, "invitation already processed", http.StatusConflict)
+		return
+	}
+
+	// Check if expired (7 days default)
+	if invitation.ExpiresAt != nil && time.Now().After(*invitation.ExpiresAt) {
+		http.Error(w, "invitation expired", http.StatusGone)
+		return
+	}
+
+	if err := h.db.AcceptInvitation(invitationID, user.Name); err != nil {
+		h.logger.Error("failed to accept invitation", "error", err)
+		http.Error(w, "failed to accept invitation", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("invitation accepted", "invitation_id", invitationID, "user", user.Name, "group_id", invitation.GroupID)
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]string{
+		"status":   "accepted",
+		"group_id": invitation.GroupID,
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// DeclineInvitation handles POST /api/invitations/{id}/decline
+func (h *Handler) DeclineInvitation(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	invitationID := r.PathValue("id")
+	if invitationID == "" {
+		http.Error(w, "invitation id required", http.StatusBadRequest)
+		return
+	}
+
+	invitation, err := h.db.GetInvitation(invitationID)
+	if err != nil {
+		http.Error(w, "invitation not found", http.StatusNotFound)
+		return
+	}
+
+	if invitation.InvitedUserID != user.Name {
+		http.Error(w, "forbidden", http.StatusForbidden)
+		return
+	}
+
+	if err := h.db.DeclineInvitation(invitationID, user.Name); err != nil {
+		h.logger.Error("failed to decline invitation", "error", err)
+		http.Error(w, "failed to decline invitation", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("invitation declined", "invitation_id", invitationID, "user", user.Name)
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]string{
+		"status": "declined",
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// RemoveMember handles DELETE /api/groups/{id}/members/{user}
+func (h *Handler) RemoveMember(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	groupID := r.PathValue("id")
+	memberID := r.PathValue("user")
+	if groupID == "" || memberID == "" {
+		http.Error(w, "group id and user id required", http.StatusBadRequest)
+		return
+	}
+
+	// Check if user is group admin
+	isAdmin, err := h.db.IsGroupAdmin(user.Name, groupID)
+	if err != nil || !isAdmin {
+		http.Error(w, "forbidden: group admin required", http.StatusForbidden)
+		return
+	}
+
+	// Cannot remove yourself (use leave group endpoint instead)
+	if memberID == user.Name {
+		http.Error(w, "cannot remove yourself; use leave group endpoint", http.StatusBadRequest)
+		return
+	}
+
+	if err := h.db.RemoveGroupMember(groupID, memberID); err != nil {
+		h.logger.Error("failed to remove member", "error", err)
+		http.Error(w, "failed to remove member", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("member removed", "group_id", groupID, "member", memberID, "removed_by", user.Name)
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// ListGroupTasks handles GET /api/groups/{id}/tasks
+func (h *Handler) ListGroupTasks(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	groupID := r.PathValue("id")
+	if groupID == "" {
+		http.Error(w, "group id required", http.StatusBadRequest)
+		return
+	}
+
+	// Check if user is member of group
+	isMember, err := h.db.IsGroupMember(user.Name, groupID)
+	if err != nil || !isMember {
+		http.Error(w, "forbidden: group membership required", http.StatusForbidden)
+		return
+	}
+
+	// Parse pagination options
+	limit := 100
+	if l := r.URL.Query().Get("limit"); l != "" {
+		if parsed, err := strconv.Atoi(l); err == nil && parsed > 0 && parsed <= 100 {
+			limit = parsed
+		}
+	}
+	cursor := r.URL.Query().Get("cursor")
+
+	opts := storage.ListTasksOptions{
+		Limit:  limit,
+		Cursor: cursor,
+	}
+
+	tasks, nextCursor, err := h.db.ListTasksForGroup(groupID, opts)
+	if err != nil {
+		h.logger.Error("failed to list group tasks", "error", err)
+		http.Error(w, "failed to list tasks", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]interface{}{
+		"tasks":       tasks,
+		"next_cursor": nextCursor,
+		"count":       len(tasks),
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// sendErrorPacket sends an error response packet to the client
+func (h *Handler) sendErrorPacket(conn *websocket.Conn, code byte, message, details string) error {
+	err := map[string]interface{}{
+		"error":   true,
+		"code":    code,
+		"message": message,
+		"details": details,
+	}
+	return conn.WriteJSON(err)
+}
+
+// sendSuccessPacket sends a success response packet
+func (h *Handler) sendSuccessPacket(conn *websocket.Conn, data map[string]interface{}) error {
+	return conn.WriteJSON(data)
+}
+
+// HandleCreateGroup handles WebSocket group creation.
+// Protocol: [api_key_hash:16][name_len:1][name:var][desc_len:2][desc:var]
+func (h *Handler) HandleCreateGroup(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+1+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	nameLen := int(payload[offset])
+	offset++
+	if nameLen <= 0 || len(payload) < offset+nameLen+2 {
+		return h.sendErrorPacket(conn, 0x01, "invalid name length", "")
+	}
+	name := string(payload[offset : offset+nameLen])
+	offset += nameLen
+
+	descLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if descLen < 0 || len(payload) < offset+descLen {
+		return h.sendErrorPacket(conn, 0x01, "invalid description length", "")
+	}
+	description := string(payload[offset : offset+descLen])
+
+	// Only admins can create groups
+	if !user.Admin {
+		return h.sendErrorPacket(conn, 0x03, "forbidden: admin required", "")
+	}
+
+	if name == "" {
+		return h.sendErrorPacket(conn, 0x01, "name is required", "")
+	}
+
+	group, err := h.db.CreateGroup(name, description, user.Name)
+	if err != nil {
+		h.logger.Error("failed to create group", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to create group", err.Error())
+	}
+
+	h.logger.Info("group created", "group_id", group.ID, "name", group.Name, "created_by", user.Name)
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success": true,
+		"group":   group,
+	})
+}
+
+// HandleListGroups handles WebSocket group listing.
+// Protocol: [api_key_hash:16]
+func (h *Handler) HandleListGroups(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	groups, err := h.db.ListGroupsForUser(user.Name)
+	if err != nil {
+		h.logger.Error("failed to list groups", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to list groups", err.Error())
+	}
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success": true,
+		"groups":  groups,
+		"count":   len(groups),
+	})
+}
+
+// HandleCreateInvitation handles WebSocket invitation creation.
+// Protocol: [api_key_hash:16][group_id_len:2][group_id:var][user_id_len:2][user_id:var]
+func (h *Handler) HandleCreateInvitation(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+2+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	groupIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if groupIDLen <= 0 || len(payload) < offset+groupIDLen+2 {
+		return h.sendErrorPacket(conn, 0x01, "invalid group_id length", "")
+	}
+	groupID := string(payload[offset : offset+groupIDLen])
+	offset += groupIDLen
+
+	userIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if userIDLen <= 0 || len(payload) < offset+userIDLen {
+		return h.sendErrorPacket(conn, 0x01, "invalid user_id length", "")
+	}
+	invitedUserID := string(payload[offset : offset+userIDLen])
+
+	// Check if user is group admin
+	isAdmin, err := h.db.IsGroupAdmin(user.Name, groupID)
+	if err != nil || !isAdmin {
+		return h.sendErrorPacket(conn, 0x03, "forbidden: group admin required", "")
+	}
+
+	invitation, err := h.db.CreateGroupInvitation(groupID, invitedUserID, user.Name)
+	if err != nil {
+		h.logger.Error("failed to create invitation", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to create invitation", err.Error())
+	}
+
+	h.logger.Info("invitation created", "invitation_id", invitation.ID, "group_id", groupID, "invited_user", invitedUserID)
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success":    true,
+		"invitation": invitation,
+	})
+}
+
+// HandleListInvitations handles WebSocket invitation listing.
+// Protocol: [api_key_hash:16]
+func (h *Handler) HandleListInvitations(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	invitations, err := h.db.ListPendingInvitationsForUser(user.Name)
+	if err != nil {
+		h.logger.Error("failed to list invitations", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to list invitations", err.Error())
+	}
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success":     true,
+		"invitations": invitations,
+		"count":       len(invitations),
+	})
+}
+
+// HandleAcceptInvitation handles WebSocket invitation acceptance.
+// Protocol: [api_key_hash:16][invitation_id_len:2][invitation_id:var]
+func (h *Handler) HandleAcceptInvitation(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	invIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if invIDLen <= 0 || len(payload) < offset+invIDLen {
+		return h.sendErrorPacket(conn, 0x01, "invalid invitation_id length", "")
+	}
+	invitationID := string(payload[offset : offset+invIDLen])
+
+	// Verify invitation belongs to this user and is pending
+	invitation, err := h.db.GetInvitation(invitationID)
+	if err != nil {
+		return h.sendErrorPacket(conn, 0x04, "invitation not found", "")
+	}
+
+	if invitation.InvitedUserID != user.Name {
+		return h.sendErrorPacket(conn, 0x03, "forbidden", "")
+	}
+
+	if invitation.Status != "pending" {
+		return h.sendErrorPacket(conn, 0x05, "invitation already processed", "")
+	}
+
+	// Check if expired (7 days default)
+	if invitation.ExpiresAt != nil && time.Now().After(*invitation.ExpiresAt) {
+		return h.sendErrorPacket(conn, 0x14, "invitation expired", "")
+	}
+
+	if err := h.db.AcceptInvitation(invitationID, user.Name); err != nil {
+		h.logger.Error("failed to accept invitation", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to accept invitation", err.Error())
+	}
+
+	h.logger.Info("invitation accepted", "invitation_id", invitationID, "user", user.Name, "group_id", invitation.GroupID)
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success":  true,
+		"status":   "accepted",
+		"group_id": invitation.GroupID,
+	})
+}
+
+// HandleDeclineInvitation handles WebSocket invitation decline.
+// Protocol: [api_key_hash:16][invitation_id_len:2][invitation_id:var]
+func (h *Handler) HandleDeclineInvitation(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	invIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if invIDLen <= 0 || len(payload) < offset+invIDLen {
+		return h.sendErrorPacket(conn, 0x01, "invalid invitation_id length", "")
+	}
+	invitationID := string(payload[offset : offset+invIDLen])
+
+	invitation, err := h.db.GetInvitation(invitationID)
+	if err != nil {
+		return h.sendErrorPacket(conn, 0x04, "invitation not found", "")
+	}
+
+	if invitation.InvitedUserID != user.Name {
+		return h.sendErrorPacket(conn, 0x03, "forbidden", "")
+	}
+
+	if err := h.db.DeclineInvitation(invitationID, user.Name); err != nil {
+		h.logger.Error("failed to decline invitation", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to decline invitation", err.Error())
+	}
+
+	h.logger.Info("invitation declined", "invitation_id", invitationID, "user", user.Name)
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success": true,
+		"status":  "declined",
+	})
+}
+
+// HandleRemoveMember handles WebSocket member removal.
+// Protocol: [api_key_hash:16][group_id_len:2][group_id:var][user_id_len:2][user_id:var]
+func (h *Handler) HandleRemoveMember(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+2+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	groupIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if groupIDLen <= 0 || len(payload) < offset+groupIDLen+2 {
+		return h.sendErrorPacket(conn, 0x01, "invalid group_id length", "")
+	}
+	groupID := string(payload[offset : offset+groupIDLen])
+	offset += groupIDLen
+
+	userIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if userIDLen <= 0 || len(payload) < offset+userIDLen {
+		return h.sendErrorPacket(conn, 0x01, "invalid user_id length", "")
+	}
+	memberID := string(payload[offset : offset+userIDLen])
+
+	// Check if user is group admin
+	isAdmin, err := h.db.IsGroupAdmin(user.Name, groupID)
+	if err != nil || !isAdmin {
+		return h.sendErrorPacket(conn, 0x03, "forbidden: group admin required", "")
+	}
+
+	// Cannot remove yourself
+	if memberID == user.Name {
+		return h.sendErrorPacket(conn, 0x01, "cannot remove yourself; use leave group endpoint", "")
+	}
+
+	if err := h.db.RemoveGroupMember(groupID, memberID); err != nil {
+		h.logger.Error("failed to remove member", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to remove member", err.Error())
+	}
+
+	h.logger.Info("member removed", "group_id", groupID, "member", memberID, "removed_by", user.Name)
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success": true,
+		"message": "Member removed",
+	})
+}
+
+// HandleListGroupTasks handles WebSocket group task listing.
+// Protocol: [api_key_hash:16][group_id_len:2][group_id:var][limit:1][cursor_len:2][cursor:var]
+func (h *Handler) HandleListGroupTasks(conn *websocket.Conn, payload []byte, user *auth.User) error {
+	if len(payload) < 16+2+1+2 {
+		return h.sendErrorPacket(conn, 0x01, "payload too short", "")
+	}
+
+	offset := 16
+
+	groupIDLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	if groupIDLen <= 0 || len(payload) < offset+groupIDLen+1+2 {
+		return h.sendErrorPacket(conn, 0x01, "invalid group_id length", "")
+	}
+	groupID := string(payload[offset : offset+groupIDLen])
+	offset += groupIDLen
+
+	// Check if user is member of group
+	isMember, err := h.db.IsGroupMember(user.Name, groupID)
+	if err != nil || !isMember {
+		return h.sendErrorPacket(conn, 0x03, "forbidden: group membership required", "")
+	}
+
+	limit := int(payload[offset])
+	offset++
+	if limit <= 0 || limit > 100 {
+		limit = 100
+	}
+
+	cursorLen := int(binary.BigEndian.Uint16(payload[offset : offset+2]))
+	offset += 2
+	var cursor string
+	if cursorLen > 0 {
+		if len(payload) < offset+cursorLen {
+			return h.sendErrorPacket(conn, 0x01, "invalid cursor length", "")
+		}
+		cursor = string(payload[offset : offset+cursorLen])
+	}
+
+	opts := storage.ListTasksOptions{
+		Limit:  limit,
+		Cursor: cursor,
+	}
+
+	tasks, nextCursor, err := h.db.ListTasksForGroup(groupID, opts)
+	if err != nil {
+		h.logger.Error("failed to list group tasks", "error", err)
+		return h.sendErrorPacket(conn, 0x11, "failed to list tasks", err.Error())
+	}
+
+	return h.sendSuccessPacket(conn, map[string]interface{}{
+		"success":     true,
+		"tasks":       tasks,
+		"next_cursor": nextCursor,
+		"count":       len(tasks),
+	})
+}
diff --git a/internal/api/jobs/handlers.go b/internal/api/jobs/handlers.go
index fdac92e..bcd564d 100644
--- a/internal/api/jobs/handlers.go
+++ b/internal/api/jobs/handlers.go
@@ -2,7 +2,6 @@
 package jobs
 
 import (
-	"context"
 	"encoding/binary"
 	"encoding/json"
 	"net/http"
@@ -22,14 +21,11 @@ import (
 
 // Handler provides job-related WebSocket handlers
 type Handler struct {
-	expManager      *experiment.Manager
-	logger          *logging.Logger
-	queue           queue.Backend
-	db              *storage.DB
-	authConfig      *auth.Config
-	privacyEnforcer interface { // NEW: Privacy enforcement interface
-		CanAccess(ctx context.Context, user *auth.User, owner string, level string, team string) (bool, error)
-	}
+	expManager *experiment.Manager
+	logger     *logging.Logger
+	queue      queue.Backend
+	db         *storage.DB
+	authConfig *auth.Config
 }
 
 // NewHandler creates a new jobs handler
@@ -39,17 +35,13 @@ func NewHandler(
 	queue queue.Backend,
 	db *storage.DB,
 	authConfig *auth.Config,
-	privacyEnforcer interface { // NEW - can be nil
-		CanAccess(ctx context.Context, user *auth.User, owner string, level string, team string) (bool, error)
-	},
 ) *Handler {
 	return &Handler{
-		expManager:      expManager,
-		logger:          logger,
-		queue:           queue,
-		db:              db,
-		authConfig:      authConfig,
-		privacyEnforcer: privacyEnforcer,
+		expManager: expManager,
+		logger:     logger,
+		queue:      queue,
+		db:         db,
+		authConfig: authConfig,
 	}
 }
 
@@ -75,7 +67,7 @@ const (
 
 // sendErrorPacket sends an error response packet to the client
 func (h *Handler) sendErrorPacket(conn *websocket.Conn, code byte, message, details string) error {
-	err := map[string]interface{}{
+	err := map[string]any{
 		"error":   true,
 		"code":    code,
 		"message": message,
@@ -85,7 +77,7 @@ func (h *Handler) sendErrorPacket(conn *websocket.Conn, code byte, message, deta
 }
 
 // sendSuccessPacket sends a success response packet
-func (h *Handler) sendSuccessPacket(conn *websocket.Conn, data map[string]interface{}) error {
+func (h *Handler) sendSuccessPacket(conn *websocket.Conn, data map[string]any) error {
 	return conn.WriteJSON(data)
 }
 
@@ -340,7 +332,7 @@ func (h *Handler) HandleListJobs(conn *websocket.Conn, user *auth.User) error {
 
 	jobPaths := storage.NewJobPaths(base)
 
-	jobs := []map[string]interface{}{}
+	jobs := []map[string]any{}
 
 	// Scan all job directories
 	for _, bucket := range []string{"running", "pending", "finished", "failed"} {
@@ -411,9 +403,9 @@ func (h *Handler) GetExperimentHistoryHTTP(w http.ResponseWriter, r *http.Reques
 	h.logger.Info("getting experiment history", "experiment", experimentID, "all_users", allUsers)
 
 	// Placeholder response
-	response := map[string]interface{}{
+	response := map[string]any{
 		"experiment_id": experimentID,
-		"history": []map[string]interface{}{
+		"history": []map[string]any{
 			{
 				"timestamp": time.Now().UTC(),
 				"event":     "run_started",
@@ -440,7 +432,9 @@ func (h *Handler) GetExperimentHistoryHTTP(w http.ResponseWriter, r *http.Reques
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(response)
+	if err := json.NewEncoder(w).Encode(response); err != nil {
+		h.logger.Warn("failed to encode compare response", "error", err)
+	}
 }
 
 // ListAllJobsHTTP handles GET /api/jobs?all_users=true for team collaboration
@@ -455,7 +449,7 @@ func (h *Handler) ListAllJobsHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	jobPaths := storage.NewJobPaths(base)
-	jobs := []map[string]interface{}{}
+	jobs := []map[string]any{}
 
 	// Scan all job directories
 	for _, bucket := range []string{"running", "pending", "finished", "failed"} {
@@ -500,7 +494,7 @@ func (h *Handler) ListAllJobsHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	response := map[string]interface{}{
+	response := map[string]any{
 		"success": true,
 		"jobs":    jobs,
 		"count":   len(jobs),
@@ -508,5 +502,7 @@ func (h *Handler) ListAllJobsHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(response)
+	if err := json.NewEncoder(w).Encode(response); err != nil {
+		h.logger.Warn("failed to encode jobs list", "error", err)
+	}
 }
diff --git a/internal/api/jupyter/handlers.go b/internal/api/jupyter/handlers.go
index a179d2b..713181c 100644
--- a/internal/api/jupyter/handlers.go
+++ b/internal/api/jupyter/handlers.go
@@ -254,3 +254,7 @@ func (h *Handler) ListServicesHTTP(w http.ResponseWriter, r *http.Request) {
 func (h *Handler) StartServiceHTTP(w http.ResponseWriter, r *http.Request) {
 	http.Error(w, "Not implemented", http.StatusNotImplemented)
 }
+
+func (h *Handler) StopServiceHTTP(w http.ResponseWriter, r *http.Request) {
+	http.Error(w, "Not mplementated", http.StatusNotImplemented)
+}
diff --git a/internal/api/routes.go b/internal/api/routes.go
index 82f852f..15ea872 100644
--- a/internal/api/routes.go
+++ b/internal/api/routes.go
@@ -6,6 +6,7 @@ import (
 
 	"github.com/jfraeys/fetch_ml/internal/api/audit"
 	"github.com/jfraeys/fetch_ml/internal/api/datasets"
+	"github.com/jfraeys/fetch_ml/internal/api/groups"
 	"github.com/jfraeys/fetch_ml/internal/api/jobs"
 	"github.com/jfraeys/fetch_ml/internal/api/jupyter"
 	"github.com/jfraeys/fetch_ml/internal/api/plugins"
@@ -46,7 +47,6 @@ func (s *Server) registerRoutes(mux *http.ServeMux) {
 		s.taskQueue,
 		s.db,
 		s.config.BuildAuthConfig(),
-		nil,
 	)
 
 	// Experiment history endpoint: GET /api/experiments/:id/history
@@ -173,7 +173,6 @@ func (s *Server) registerWebSocketRoutes(mux *http.ServeMux) {
 		s.taskQueue,
 		s.db,
 		s.config.BuildAuthConfig(),
-		nil, // privacyEnforcer - not enabled for now
 	)
 
 	// Create jupyter handler
@@ -190,6 +189,9 @@ func (s *Server) registerWebSocketRoutes(mux *http.ServeMux) {
 		s.config.DataDir,
 	)
 
+	// Create groups handler
+	groupsHandler := groups.NewHandler(s.db, s.logger)
+
 	wsHandler := ws.NewHandler(
 		s.config.BuildAuthConfig(),
 		s.logger,
@@ -203,6 +205,7 @@ func (s *Server) registerWebSocketRoutes(mux *http.ServeMux) {
 		jobsHandler,
 		jupyterHandler,
 		datasetsHandler,
+		groupsHandler,
 	)
 
 	mux.Handle("/ws", wsHandler)
diff --git a/internal/api/scheduler/handlers.go b/internal/api/scheduler/handlers.go
index e6eb7f9..ce7bbf5 100644
--- a/internal/api/scheduler/handlers.go
+++ b/internal/api/scheduler/handlers.go
@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
-	"strconv"
 	"time"
 
 	"github.com/jfraeys/fetch_ml/internal/auth"
@@ -124,7 +123,9 @@ func (h *APIHandler) GetV1SchedulerStatus(w http.ResponseWriter, r *http.Request
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(status)
+	if err := json.NewEncoder(w).Encode(status); err != nil {
+		h.logger.Warn("failed to encode status", "error", err)
+	}
 }
 
 // GetV1SchedulerWorkers handles GET /v1/scheduler/workers
@@ -159,7 +160,9 @@ func (h *APIHandler) GetV1SchedulerWorkers(w http.ResponseWriter, r *http.Reques
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(workers)
+	if err := json.NewEncoder(w).Encode(workers); err != nil {
+		h.logger.Warn("failed to encode workers", "error", err)
+	}
 }
 
 // GetV1SchedulerWorkersWorkerID handles GET /v1/scheduler/workers/{workerId}
@@ -200,7 +203,9 @@ func (h *APIHandler) GetV1SchedulerWorkersWorkerID(w http.ResponseWriter, r *htt
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(worker)
+	if err := json.NewEncoder(w).Encode(worker); err != nil {
+		h.logger.Warn("failed to encode worker", "error", err)
+	}
 }
 
 // DeleteV1SchedulerWorkersWorkerID handles DELETE /v1/scheduler/workers/{workerId}
@@ -247,7 +252,9 @@ func (h *APIHandler) GetV1SchedulerReservations(w http.ResponseWriter, r *http.R
 	reservations := []ReservationInfo{}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(reservations)
+	if err := json.NewEncoder(w).Encode(reservations); err != nil {
+		h.logger.Warn("failed to encode reservations", "error", err)
+	}
 }
 
 // PostV1SchedulerReservations handles POST /v1/scheduler/reservations
@@ -295,7 +302,9 @@ func (h *APIHandler) PostV1SchedulerReservations(w http.ResponseWriter, r *http.
 
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusCreated)
-	json.NewEncoder(w).Encode(reservation)
+	if err := json.NewEncoder(w).Encode(reservation); err != nil {
+		h.logger.Warn("failed to encode reservation", "error", err)
+	}
 }
 
 // PatchV1SchedulerJobsJobIDPriority handles PATCH /v1/scheduler/jobs/{jobId}/priority
@@ -342,7 +351,9 @@ func (h *APIHandler) PatchV1SchedulerJobsJobIDPriority(w http.ResponseWriter, r
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(response)
+	if err := json.NewEncoder(w).Encode(response); err != nil {
+		h.logger.Warn("failed to encode priority response", "error", err)
+	}
 }
 
 // GetV1SchedulerStatusStream handles GET /v1/scheduler/status/stream (SSE)
@@ -465,16 +476,3 @@ func (h *APIHandler) checkPermission(user *auth.User, permission string) bool {
 	// Check specific permission
 	return user.HasPermission(permission)
 }
-
-// parseIntQueryParam parses an integer query parameter
-func parseIntQueryParam(r *http.Request, name string, defaultVal int) int {
-	str := r.URL.Query().Get(name)
-	if str == "" {
-		return defaultVal
-	}
-	val, err := strconv.Atoi(str)
-	if err != nil {
-		return defaultVal
-	}
-	return val
-}
diff --git a/internal/api/tokens/handlers.go b/internal/api/tokens/handlers.go
new file mode 100644
index 0000000..0996b77
--- /dev/null
+++ b/internal/api/tokens/handlers.go
@@ -0,0 +1,197 @@
+// Package tokens provides HTTP handlers for share token management
+package tokens
+
+import (
+	"encoding/json"
+	"net/http"
+	"time"
+
+	"github.com/jfraeys/fetch_ml/internal/auth"
+	"github.com/jfraeys/fetch_ml/internal/logging"
+	"github.com/jfraeys/fetch_ml/internal/storage"
+)
+
+// Handler provides share token management HTTP handlers
+type Handler struct {
+	db     *storage.DB
+	logger *logging.Logger
+}
+
+// NewHandler creates a new share tokens handler
+func NewHandler(db *storage.DB, logger *logging.Logger) *Handler {
+	return &Handler{
+		db:     db,
+		logger: logger,
+	}
+}
+
+// CreateTokenRequest represents a request to create a new share token
+type CreateTokenRequest struct {
+	TaskID       *string `json:"task_id,omitempty"`
+	ExperimentID *string `json:"experiment_id,omitempty"`
+	ExpiresIn    *int    `json:"expires_in_days,omitempty"` // Number of days until expiry
+	MaxAccesses  *int    `json:"max_accesses,omitempty"`    // Max number of accesses allowed
+}
+
+// CreateTokenResponse represents the response with the created token
+type CreateTokenResponse struct {
+	Token     string `json:"token"`
+	ShareLink string `json:"share_link"`
+}
+
+// CreateShareToken handles POST /api/tokens
+// Creates a new share token for a task or experiment
+func (h *Handler) CreateShareToken(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	// Check if user has permission to share tasks
+	if !user.HasPermission(auth.PermissionTasksShare) {
+		http.Error(w, "forbidden: insufficient permissions to share tasks", http.StatusForbidden)
+		return
+	}
+
+	var req CreateTokenRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		http.Error(w, "invalid request body", http.StatusBadRequest)
+		return
+	}
+
+	// Must specify either task_id or experiment_id, not both
+	if (req.TaskID == nil && req.ExperimentID == nil) || (req.TaskID != nil && req.ExperimentID != nil) {
+		http.Error(w, "must specify exactly one of task_id or experiment_id", http.StatusBadRequest)
+		return
+	}
+
+	// Build options
+	opts := auth.ShareTokenOptions{}
+	if req.ExpiresIn != nil {
+		expiresAt := time.Now().Add(time.Duration(*req.ExpiresIn) * 24 * time.Hour)
+		opts.ExpiresAt = &expiresAt
+	}
+	if req.MaxAccesses != nil {
+		opts.MaxAccesses = req.MaxAccesses
+	}
+
+	// Generate token
+	token, err := auth.GenerateShareToken(h.db, req.TaskID, req.ExperimentID, user.Name, opts)
+	if err != nil {
+		h.logger.Error("failed to generate share token", "error", err)
+		http.Error(w, "failed to create share token", http.StatusInternalServerError)
+		return
+	}
+
+	// Build share link
+	var path string
+	if req.TaskID != nil {
+		path = "/api/tasks/" + *req.TaskID
+	} else {
+		path = "/api/experiments/" + *req.ExperimentID
+	}
+	shareLink := auth.BuildShareLink("", path, token)
+
+	h.logger.Info("share token created",
+		"token", token[:8]+"...",
+		"created_by", user.Name,
+		"task_id", req.TaskID,
+		"experiment_id", req.ExperimentID,
+	)
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusCreated)
+	if err := json.NewEncoder(w).Encode(CreateTokenResponse{
+		Token:     token,
+		ShareLink: shareLink,
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// ListShareTokens handles GET /api/tokens
+// Lists share tokens for a task or experiment
+func (h *Handler) ListShareTokens(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	taskID := r.URL.Query().Get("task_id")
+	experimentID := r.URL.Query().Get("experiment_id")
+
+	// Must specify either task_id or experiment_id
+	if taskID == "" && experimentID == "" {
+		http.Error(w, "must specify task_id or experiment_id", http.StatusBadRequest)
+		return
+	}
+
+	var tokens []*storage.ShareToken
+	var err error
+
+	if taskID != "" {
+		tokens, err = h.db.ListShareTokensForTask(taskID)
+	} else {
+		tokens, err = h.db.ListShareTokensForExperiment(experimentID)
+	}
+
+	if err != nil {
+		h.logger.Error("failed to list share tokens", "error", err)
+		http.Error(w, "failed to list share tokens", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	if err := json.NewEncoder(w).Encode(map[string]any{
+		"tokens": tokens,
+		"count":  len(tokens),
+	}); err != nil {
+		h.logger.Error("failed to encode response", "error", err)
+	}
+}
+
+// RevokeShareToken handles DELETE /api/tokens/:token
+// Revokes a share token
+func (h *Handler) RevokeShareToken(w http.ResponseWriter, r *http.Request) {
+	user := auth.GetUserFromContext(r.Context())
+	if user == nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	token := r.PathValue("token")
+	if token == "" {
+		http.Error(w, "token required", http.StatusBadRequest)
+		return
+	}
+
+	// Get token details to check ownership
+	t, err := h.db.GetShareToken(token)
+	if err != nil {
+		h.logger.Error("failed to get share token", "error", err)
+		http.Error(w, "token not found", http.StatusNotFound)
+		return
+	}
+	if t == nil {
+		http.Error(w, "token not found", http.StatusNotFound)
+		return
+	}
+
+	// Only the creator or an admin can revoke
+	if t.CreatedBy != user.Name && !user.Admin {
+		http.Error(w, "forbidden: not token owner or admin", http.StatusForbidden)
+		return
+	}
+
+	if err := h.db.DeleteShareToken(token); err != nil {
+		h.logger.Error("failed to delete share token", "error", err)
+		http.Error(w, "failed to revoke token", http.StatusInternalServerError)
+		return
+	}
+
+	h.logger.Info("share token revoked", "token", token[:8]+"...", "revoked_by", user.Name)
+
+	w.WriteHeader(http.StatusNoContent)
+}
diff --git a/internal/api/ws/handler.go b/internal/api/ws/handler.go
index 3de15da..2937578 100644
--- a/internal/api/ws/handler.go
+++ b/internal/api/ws/handler.go
@@ -27,6 +27,7 @@ import (
 	"github.com/jfraeys/fetch_ml/internal/storage"
 
 	"github.com/jfraeys/fetch_ml/internal/api/datasets"
+	"github.com/jfraeys/fetch_ml/internal/api/groups"
 	"github.com/jfraeys/fetch_ml/internal/api/jobs"
 	jupyterj "github.com/jfraeys/fetch_ml/internal/api/jupyter"
 )
@@ -75,6 +76,16 @@ const (
 	OpcodeQueryJob     = 0x23
 	OpcodeQueryRunInfo = 0x28
 
+	// Group management opcodes
+	OpcodeCreateGroup       = 0x50
+	OpcodeListGroups        = 0x51
+	OpcodeCreateInvitation  = 0x52
+	OpcodeListInvitations   = 0x53
+	OpcodeAcceptInvitation  = 0x54
+	OpcodeDeclineInvitation = 0x55
+	OpcodeRemoveMember      = 0x56
+	OpcodeListGroupTasks    = 0x57
+
 	//
 	OpcodeCompareRuns   = 0x30
 	OpcodeFindRuns      = 0x31
@@ -147,6 +158,7 @@ type Handler struct {
 	authConfig        *auth.Config
 	jobsHandler       *jobs.Handler
 	jupyterHandler    *jupyterj.Handler
+	groupsHandler     *groups.Handler // NEW: groups handler for WebSocket
 	upgrader          websocket.Upgrader
 	dataDir           string
 	clientsMu         sync.RWMutex
@@ -166,6 +178,7 @@ func NewHandler(
 	jobsHandler *jobs.Handler,
 	jupyterHandler *jupyterj.Handler,
 	datasetsHandler *datasets.Handler,
+	groupsHandler *groups.Handler,
 ) *Handler {
 	upgrader := createUpgrader(securityCfg)
 
@@ -183,6 +196,7 @@ func NewHandler(
 		jobsHandler:       jobsHandler,
 		jupyterHandler:    jupyterHandler,
 		datasetsHandler:   datasetsHandler,
+		groupsHandler:     groupsHandler,
 		clients:           make(map[*Client]bool),
 	}
 }
@@ -339,6 +353,22 @@ func (h *Handler) handleMessage(conn *websocket.Conn, payload []byte) error {
 		return h.handleSetRunOutcome(conn, payload)
 	case OpcodeQueryRunInfo:
 		return h.handleQueryRunInfo(conn, payload)
+	case OpcodeCreateGroup:
+		return h.handleCreateGroup(conn, payload)
+	case OpcodeListGroups:
+		return h.handleListGroups(conn, payload)
+	case OpcodeCreateInvitation:
+		return h.handleCreateInvitation(conn, payload)
+	case OpcodeListInvitations:
+		return h.handleListInvitations(conn, payload)
+	case OpcodeAcceptInvitation:
+		return h.handleAcceptInvitation(conn, payload)
+	case OpcodeDeclineInvitation:
+		return h.handleDeclineInvitation(conn, payload)
+	case OpcodeRemoveMember:
+		return h.handleRemoveMember(conn, payload)
+	case OpcodeListGroupTasks:
+		return h.handleListGroupTasks(conn, payload)
 	default:
 		return h.sendErrorPacket(conn, ErrorCodeInvalidRequest, "unknown opcode", string(opcode))
 	}
@@ -429,6 +459,54 @@ func (h *Handler) handleListJupyter(conn *websocket.Conn, payload []byte) error
 	})
 }
 
+func (h *Handler) handleCreateGroup(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleCreateGroup(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleListGroups(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleListGroups(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleCreateInvitation(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleCreateInvitation(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleListInvitations(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleListInvitations(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleAcceptInvitation(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleAcceptInvitation(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleDeclineInvitation(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleDeclineInvitation(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleRemoveMember(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleRemoveMember(conn, payload, user)
+	})
+}
+
+func (h *Handler) handleListGroupTasks(conn *websocket.Conn, payload []byte) error {
+	return h.withAuth(conn, payload, func(user *auth.User) error {
+		return h.groupsHandler.HandleListGroupTasks(conn, payload, user)
+	})
+}
+
 func (h *Handler) handleLogMetric(conn *websocket.Conn, payload []byte) error {
 	// Parse payload: [api_key_hash:16][metric_name_len:1][metric_name:var][value:8]
 	if len(payload) < 16+1+8 {
@@ -590,9 +668,15 @@ func selectDependencyManifest(filesPath string) (string, error) {
 	for _, name := range []string{
 		"requirements.txt", "package.json", "Cargo.toml", "go.mod", "pom.xml", "build.gradle",
 	} {
-		if _, err := os.Stat(filepath.Join(filesPath, name)); err == nil {
+		//nolint:gosec // G703: name is from hardcoded slice, not user input
+		fullPath := filepath.Join(filesPath, name)
+		_, err := os.Stat(fullPath)
+		if err == nil {
 			return name, nil
 		}
+		if !os.IsNotExist(err) {
+			return "", fmt.Errorf("error checking %s: %w", name, err)
+		}
 	}
 	return "", fmt.Errorf("no dependency manifest found")
 }
diff --git a/internal/auth/api_key.go b/internal/auth/api_key.go
index 9d8afbf..1eadfc4 100644
--- a/internal/auth/api_key.go
+++ b/internal/auth/api_key.go
@@ -19,6 +19,7 @@ type User struct {
 	Name        string          `json:"name"`
 	Roles       []string        `json:"roles"`
 	Admin       bool            `json:"admin"`
+	Groups      []string        `json:"groups"` // NEW: lab groups ("ml-lab", "nlp-group")
 }
 
 // ExtractAPIKeyFromRequest extracts an API key from the standard headers.
diff --git a/internal/auth/database.go b/internal/auth/database.go
index 69d305a..1c36342 100644
--- a/internal/auth/database.go
+++ b/internal/auth/database.go
@@ -194,7 +194,11 @@ func (s *DatabaseAuthStore) ListUsers(ctx context.Context) ([]APIKeyRecord, erro
 	if err != nil {
 		return nil, fmt.Errorf("failed to query users: %w", err)
 	}
-	defer func() { _ = rows.Close() }()
+	defer func() {
+		if err := rows.Close(); err != nil {
+			log.Printf("ERROR: failed to close rows: %v", err)
+		}
+	}()
 
 	var users []APIKeyRecord
 	for rows.Next() {
diff --git a/internal/auth/flags.go b/internal/auth/flags.go
index 277906c..88dce52 100644
--- a/internal/auth/flags.go
+++ b/internal/auth/flags.go
@@ -4,6 +4,7 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"log/slog"
 	"os"
 	"strings"
 
@@ -52,7 +53,8 @@ func GetAPIKeyFromSources(flags *Flags) string {
 		if readErr == nil {
 			return strings.TrimSpace(string(contents))
 		}
-		log.Printf("Warning: Could not read API key file %s: %v", flags.APIKeyFile, readErr)
+		// Use structured logging to prevent log injection
+		slog.Warn("Could not read API key file", "file", flags.APIKeyFile, "error", readErr)
 	}
 
 	// 3. Environment variable
@@ -66,7 +68,8 @@ func GetAPIKeyFromSources(flags *Flags) string {
 		if err == nil {
 			return strings.TrimSpace(string(content))
 		}
-		log.Printf("Warning: Could not read API key file %s: %v", fileKey, err)
+		// Use structured logging to prevent log injection
+		slog.Warn("Could not read API key file", "file", fileKey, "error", err)
 	}
 
 	return ""
@@ -84,7 +87,8 @@ func ValidateFlags(flags *Flags) error {
 			return fmt.Errorf("api key file not found: %s", flags.APIKeyFile)
 		}
 		if err := CheckConfigFilePermissions(flags.APIKeyFile); err != nil {
-			log.Printf("Warning: %v", err)
+			// Use structured logging to prevent log injection
+			slog.Warn("API key file permission issue", "error", err)
 		}
 	}
 
@@ -96,7 +100,8 @@ func ValidateFlags(flags *Flags) error {
 
 		// Check file permissions
 		if err := CheckConfigFilePermissions(flags.ConfigFile); err != nil {
-			log.Printf("Warning: %v", err)
+			// Use structured logging to prevent log injection
+			slog.Warn("Config file permission issue", "error", err)
 		}
 	}
 
diff --git a/internal/auth/permissions.go b/internal/auth/permissions.go
index 63149b3..ac6a33c 100644
--- a/internal/auth/permissions.go
+++ b/internal/auth/permissions.go
@@ -3,6 +3,8 @@ package auth
 import (
 	"fmt"
 	"strings"
+
+	"github.com/jfraeys/fetch_ml/internal/domain"
 )
 
 // Permission constants for type safety
@@ -31,6 +33,13 @@ const (
 	PermissionSystemLogs    = "system:logs"
 	PermissionSystemUsers   = "system:users"
 
+	// Task sharing permissions
+	PermissionTasksReadOwn = "tasks:read:own"
+	PermissionTasksReadLab = "tasks:read:lab"
+	PermissionTasksReadAll = "tasks:read:all"
+	PermissionTasksShare   = "tasks:share"
+	PermissionTasksClone   = "tasks:clone"
+
 	// Wildcard permission
 	PermissionAll = "*"
 )
@@ -210,3 +219,59 @@ func (u *User) GetEffectivePermissions() []string {
 
 	return permissions
 }
+
+// TaskAccessChecker defines the interface for checking task access permissions.
+// Implemented by storage.DB to avoid circular imports.
+type TaskAccessChecker interface {
+	TaskIsSharedWithUser(taskID, userID string) bool
+	UserSharesGroupWithTask(userID, taskID string) bool
+	TaskAllowsPublicClone(taskID string) bool
+	UserRoleInTaskGroups(userID, taskID string) string // NEW: returns highest role (admin/member/viewer)
+}
+
+// CanAccessTask checks if an authenticated user can access a specific task.
+// For unauthenticated (token-based) access, use CanAccessWithToken() instead.
+// NOTE: This method makes 1-2 DB calls per invocation. Never call it in a loop
+// over a task list — use ListTasksForUser() instead, which resolves all access
+// in a single query.
+func (u *User) CanAccessTask(task *domain.Task, db TaskAccessChecker) bool {
+	if task.UserID == u.Name || task.CreatedBy == u.Name {
+		return true
+	}
+	if u.Admin {
+		return true
+	}
+	// Explicit share — expiry checked in SQL
+	if db.TaskIsSharedWithUser(task.ID, u.Name) {
+		return true
+	}
+	switch task.Visibility {
+	case domain.VisibilityPrivate:
+		return false
+	case domain.VisibilityLab:
+		return db.UserSharesGroupWithTask(u.Name, task.ID)
+	case domain.VisibilityInstitution, domain.VisibilityOpen:
+		// open tasks are accessible to authenticated users too, not only token holders
+		return u.HasPermission(PermissionTasksReadLab)
+	}
+	return false
+}
+
+// CanCloneTask checks if a user can clone (reproduce) a task.
+// Clone is a distinct action from read. Viewing a task does not imply
+// permission to reproduce it under a different account.
+func (u *User) CanCloneTask(task *domain.Task, db TaskAccessChecker) bool {
+	if !u.CanAccessTask(task, db) {
+		return false
+	}
+	if task.Visibility == domain.VisibilityOpen {
+		return db.TaskAllowsPublicClone(task.ID)
+	}
+	// NEW: viewer-role members can see but not clone
+	// role == "" means access is via explicit task_shares (not group) - allow clone
+	role := db.UserRoleInTaskGroups(u.Name, task.ID)
+	if role == "viewer" {
+		return false
+	}
+	return u.HasPermission(PermissionTasksClone)
+}
diff --git a/internal/auth/tokens.go b/internal/auth/tokens.go
new file mode 100644
index 0000000..430b0a8
--- /dev/null
+++ b/internal/auth/tokens.go
@@ -0,0 +1,64 @@
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"time"
+
+	"github.com/jfraeys/fetch_ml/internal/storage"
+)
+
+// ShareTokenOptions provides options for creating share tokens
+type ShareTokenOptions struct {
+	ExpiresAt   *time.Time // nil = never expires
+	MaxAccesses *int       // nil = unlimited
+}
+
+// GenerateShareToken creates a new cryptographically random share token.
+// Returns the token string that can be shared with others.
+func GenerateShareToken(db *storage.DB, taskID, experimentID *string, createdBy string, opts ShareTokenOptions) (string, error) {
+	// Generate 32 bytes of random data
+	raw := make([]byte, 32)
+	if _, err := rand.Read(raw); err != nil {
+		return "", fmt.Errorf("failed to generate random token: %w", err)
+	}
+
+	// Encode as base64url (URL-safe, no padding)
+	token := base64.RawURLEncoding.EncodeToString(raw)
+
+	// Store in database
+	if err := db.CreateShareToken(token, taskID, experimentID, createdBy, opts.ExpiresAt, opts.MaxAccesses); err != nil {
+		return "", fmt.Errorf("failed to store share token: %w", err)
+	}
+
+	return token, nil
+}
+
+// CanAccessWithToken checks if a token grants access to a task or experiment.
+// This is called for unauthenticated access via signed share links.
+// Returns true if the token is valid, not expired, and within access limits.
+func CanAccessWithToken(db *storage.DB, token string, taskID, experimentID *string) bool {
+	t, err := db.ValidateShareToken(token, taskID, experimentID)
+	if err != nil {
+		return false
+	}
+	if t == nil {
+		return false
+	}
+
+	// Increment access count
+	if err := db.IncrementTokenAccessCount(token); err != nil {
+		// Log error but don't fail the request - access was already validated
+		// This is a best-effort operation for tracking purposes
+		fmt.Printf("WARNING: failed to increment token access count: %v\n", err)
+	}
+
+	return true
+}
+
+// BuildShareLink constructs a shareable URL from a token.
+// The path should be the base API path (e.g., "/api/tasks/123" or "/api/experiments/456").
+func BuildShareLink(baseURL, path, token string) string {
+	return fmt.Sprintf("%s%s?token=%s", baseURL, path, token)
+}