docs: public IdentityOperationResult createMembership(String userId, String groupId) {
      checkAuthorization(Permissions.CREATE, Resources.GROUP_MEMBERSHIP, groupId);
      UserEntity user = findUserById(userId);
      GroupEntity group = findGroupById(groupId);
      MembershipEntity membership = new MembershipEntity();
      membership.setUser(user);
      membership.setGroup(group);
      getDbEntityManager().insert(membership);
      createDefaultMembershipAuthorizations(userId, groupId);
      return new IdentityOperationResult(null, IdentityOperationResult.OPERATION_CREATE);
    }