docs: @Nonnull
    public EChange assignUserToUserGroup (@Nullable final String sUserGroupID, @Nonnull @Nonempty final String sUserID)
    {
      // Resolve user group
      final UserGroup aUserGroup = getOfID (sUserGroupID);
      if (aUserGroup == null)
      {
        AuditHelper.onAuditModifyFailure (UserGroup.OT, sUserGroupID, "no-such-usergroup-id", "assign-user");
        return EChange.UNCHANGED;
      }
  
      m_aRWLock.writeLock ().lock ();
      try
      {
        if (aUserGroup.assignUser (sUserID).isUnchanged ())
          return EChange.UNCHANGED;
  
        BusinessObjectHelper.setLastModificationNow (aUserGroup);
        internalUpdateItem (aUserGroup);
      }
      finally
      {
        m_aRWLock.writeLock ().unlock ();
      }
      AuditHelper.onAuditModifySuccess (UserGroup.OT, "assign-user", sUserGroupID, sUserID);
  
      // Execute callback as the very last action
      m_aCallbacks.forEach (aCB -> aCB.onUserGroupUserAssignment (aUserGroup, sUserID, true));
  
      return EChange.CHANGED;
    }