Как добавить обработчик onmessage для плагина strophe muc

Вы можете проверить типы сообщений в вашем общем обработчике сообщений:

connection.addHandler(onMessage, null, 'message', null, null, null);

...

function onMessage(msg) {
  var to = msg.getAttribute('to');
  var from = msg.getAttribute('from');
  var type = msg.getAttribute('type');
  var elems = msg.getElementsByTagName('body');

  if (type == "chat" && elems.length > 0) {
    var body = elems[0];
    console.log('CHAT: I got a message from ' + from + ': ' + Strophe.getText(body));
  } else if (type == "groupchat" && elems.length > 0) {
    var body = elems[0];
    var room = Strophe.unescapeNode(Strophe.getNodeFromJid(from));
    var nick = Strophe.getResourceFromJid(from);
    console.log('GROUP CHAT: I got a message from ' + nick + ': ' + Strophe.getText(body) + ' in room: ' + room);
  }
  // we must return true to keep the handler alive.  
  // returning false would remove it after it finishes.
  return true;
}