You can create an array on the server, and store custom objects on it. For example, you could store the id created by Socket.io and a custom ID sent by each client to the server:

var util = require(util),

    io = require('/socket.io').listen(8080),

    fs = require('fs'),

    os = require('os'),

    url = require('url');



    var clients =[];



    io.sockets.on('connection', function (socket) {



        socket.on('storeClientInfo', function (data) {



            var clientInfo = new Object();

            clientInfo.customId         = data.customId;

            clientInfo.clientId     = socket.id;

            clients.push(clientInfo);

        });



        socket.on('disconnect', function (data) {



            for( var i=0, len=clients.length; i<len; ++i ){

                var c = clients[i];



                if(c.clientId == socket.id){

                    clients.splice(i,1);

                    break;

                }

            }



        });

    });

in this example, you need to call storeClientInfo from each client.

<script>

    var socket = io.connect('http://localhost', {port: 8080});



    socket.on('connect', function (data) {

        socket.emit('storeClientInfo', { customId:000CustomIdHere0000 });

    });

</script>

Hope this helps.