Need help implementing a chat site?

[Daniel N]

I have a little self-imposed project that I'm completely stuck on.. It's an overly simplified chat site.. Basically, you sign up, log in, and then you can see a list of all other members of the site who are currently logged in (Meaning they've logged in and not logged out, not that they're necessarily active). When you click on one, a chatbox comes up (I'd rather have it built into the page, FB style, but a JavaScript pop-up is okay too), and you can chat with them.

So far, I've implemented the sign up, log in, and online members list. My database has three tables: one for users, one for online users, and one for messages.

My main problem is this: How do I make it so that when user1 clicks on user2, a chatbox appears on user2's computer?

I'd appreciate any help .