I suspect you'll find that the basic unix chat programs are already installed.
These are unlikely to be welcome by your users though - the trick is to set up a chat server.
If you use Jabber, you'll be able to set up an AIM-like chat server.
Please understand that it is unusual for people here to provide the kind of instruction you want where there are already pages and pages of information for newbies. Have a look at:
http://www.lockergnome.com/linux/200...jabber-server/