code: public static String getPrimaryGroupName(String userName, AlluxioConfiguration conf)
        throws IOException {
      List<String> groups = getGroups(userName, conf);
      return (groups != null && groups.size() > 0) ? groups.get(0) : "";
    }
docs: Gets the primary group name of a user.
  
  @param userName Alluxio user name
  @param conf Alluxio configuration
  @return primary group name