Returns the caller's saved custom activity groups (created and ordered in the web app), each with its ordered item references. Read-only: clients bucket the unified activity list into these groups for a "My groups" view. Pass `targetUserId` to read another user's groups (requires `user_schedule:update` / `user_schedule:read_all`). Membership and ordering are changed via the `/groups/items` endpoints.
Query parameters
targetUserIdstringOptional
Another internal user whose groups to read (requires elevated schedule permission). Omit for self.