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