docs: def group_add_user_action(model, request):
      """Add user to group.
      """
      user_id = request.params.get('id')
      if not user_id:
          user_ids = request.params.getall('id[]')
      else:
          user_ids = [user_id]
      try:
          group = model.model
          validate_add_users_to_groups(model, user_ids, [group.id])
          for user_id in user_ids:
              group.add(user_id)
          group()
          model.parent.invalidate(group.name)
          localizer = get_localizer(request)
          message = localizer.translate(_(
              'added_user_to_group',
              default="Added user '${uid}' to group '${gid}'.",
              mapping={
                  'uid': ', '.join(user_ids),
                  'gid': group.id
              }
          ))
          return {
              'success': True,
              'message': message
          }
      except ManageMembershipError as e:
          if e.reason is not LM_TARGET_UID_NOT_ALLOWED:
              raise Exception(u"Unknown ManageMembershipError reason.")
          localizer = get_localizer(request)
          message = localizer.translate(_(
              'lm_add_target_uid_not_allowed',
              default=(
                  "Failed adding user '${uid}' to group '${gid}'. "
                  "Manage membership denied for user."
              ),
              mapping={
                  'uid': e.data,
                  'gid': group.id
              }
          ))
          return {
              'success': False,
              'message': message
          }
      except Exception as e:
          return {
              'success': False,
              'message': str(e)
          }