Finding IP of user in chat room.

I use [url]www.teenchat.com[/url]

when i log in people tell me what my IP is and what machine i am running.

they also tell me that they have added a small code in the site's java script code somehow and so they know it.

what they do is for real. and all i want to know is:
*- how do you modify the original javascript and after adding your own bit to it, how to you run it successfully and legally?

2- what is the algorithm or steps for finding the IP of the user in a chatroom whihc is based on javascrit only.

thanks.