distinctive syntax of javascript

You can create an array object of user collection as

var users = {};

then on server side you can add it as new user when you connect

socket.on('new-user', function (username) {
    users[username] = username;
});

while displaying the users, you can loop the "users" object

On Client side

var socket = io.connect();

socket.on('connect', function () {
    socket.emit('new-user', 'username');
});

http://stackoverflow.com/questions/18093638/socket-io-rooms-get-list-of-clients-in-specific-room