What would be the best way to have the user's address domain be the site they appear to be on while logged in?

i.e. The popular setup is to have them log into any domain available from thisdomain.com, and even if their address is joe@thatdomain.com the url is going to be thisdomain.com in the address bar.

That make sense? Is there a way to make it look like people are logged into the site that reflects their address without having multiple instances of GM?

If it's relative, mine are set up as virtual servers on the same box, but I still want it to appear as if they're on different domains. Would it have something to do with httpd.conf? I'd like to be able to keep /index.html's on each domain different. Thanks in advance!