@Override
  public RSGroupInfo getRSGroupOfServer(Address serverHostPort) {
    for (RSGroupInfo info : holder.groupName2Group.values()) {
      if (info.containsServer(serverHostPort)) {
        return info;
      }
    }
    return null;
  }
Returns Set of online Servers named for their hostname and port (not ServerName).
hbase-server/src/main/java/org/apache/hadoop/hbase/rsgroup/RSGroupInfoManagerImpl.java