title: Socket.IO messaging to multiple rooms
body: <p>I'm using Socket.IO in my Node Express app, and using the methods described in <a href="http://www.danielbaulig.de/socket-ioexpress/" rel="noreferrer">this excellent post</a> to relate my socket connections and sessions.  In <a href="http://www.danielbaulig.de/socket-ioexpress#div-comment-1158" rel="noreferrer">a comment</a> the author describes a way to send messages to a particular user (session) like this:</p>
  
  <pre><code>sio.on('connection', function (socket) {
      // do all the session stuff
      socket.join(socket.handshake.sessionID);
      // socket.io will leave the room upon disconnect
  });
  
  app.get('/', function (req, res) {
      sio.sockets.in(req.sessionID).send('Man, good to see you back!');
  });
  </code></pre>
  
  <p>Seems like a good idea.  However, in my app I will often by sending messages to multiple users at once.  I'm wondering about the best way to do this in Socket.IO - essentially I need to send messages to multiple rooms with the best performance possible.  Any suggestions?</p>