A simple example of setting-up dynamic "rooms" for socket.io clients to join

client.js

// set-up a connection between the client and the server
var socket = io.connect();

// let's assume that the client page, once rendered, knows what room it wants to join
var room = "abc123";

socket.on('connect', function() {
   // Connected, let's sign-up for to receive messages for this room
   socket.emit('room', room);
});

socket.on('message', function(data) {
   console.log('Incoming message:', data);
});

server.js

// attach Socket.io to our HTTP server
io = socketio.listen(server);

// handle incoming connections from clients
io.sockets.on('connection', function(socket) {
    // once a client has connected, we expect to get a ping from them saying what room they want to join
    socket.on('room', function(room) {
        socket.join(room);
    });
});

// now, it's easy to send a message to just the clients in a given room
room = "abc123";
io.sockets.in(room).emit('message', 'what is going on, party people?');

// this message will NOT go to the client defined above
io.sockets.in('foobar').emit('message', 'anyone in this room yet?');

socket.emit('message', "this is a test"); //sending to sender-client only
socket.broadcast.emit('message', "this is a test"); //sending to all clients except sender
socket.broadcast.to('game').emit('message', 'nice game'); //sending to all clients in 'game' room(channel) except sender
socket.to('game').emit('message', 'enjoy the game'); //sending to sender client, only if they are in 'game' room(channel)
socket.broadcast.to(socketid).emit('message', 'for your eyes only'); //sending to individual socketid
io.emit('message', "this is a test"); //sending to all clients, include sender
io.in('game').emit('message', 'cool game'); //sending to all clients in 'game' room(channel), include sender
io.of('myNamespace').emit('message', 'gg'); //sending to all clients in namespace 'myNamespace', include sender
socket.emit(); //send to all connected clients
socket.broadcast.emit(); //send to all connected clients except the one that sent the message
socket.on(); //event listener, can be called on client to execute on server
io.sockets.socket(); //for emiting to specific clients
io.sockets.emit(); //send to all connected clients (same as socket.emit)
io.sockets.on() ; //initial connection from a client.

// sending to spesific user with same topic
io.to(socket_id).emit('user_online', {say:"hello"});