code: public GitlabGroupMember addGroupMember(Integer groupId, Integer userId, GitlabAccessLevel accessLevel) throws IOException {
          Query query = new Query()
                  .appendIf("id", groupId)
                  .appendIf("user_id", userId)
                  .appendIf("access_level", accessLevel);
          String tailUrl = GitlabGroup.URL + "/" + groupId + GitlabProjectMember.URL + query.toString();
          return dispatch().to(tailUrl, GitlabGroupMember.class);
      }
docs: Add a group member.
  
  @param groupId     the group id
  @param userId      the user id
  @param accessLevel the GitlabAccessLevel
  @return the GitlabGroupMember
  @throws IOException on gitlab api call error