Concurrent users?!

I have a job where I need users to login to a website; that I've done and is no problem, but I need it to do one other thing:

I need to retain a list of currently logged in users (with a timeout of about 20minutes or so) and be able to deny logins to the website if the user account is already logged in.

I've tried Session vars but those are not accessable to anyone other that the client they are for.

I've tried Application vars, but there is no way to 'link' a certain client to a application variable.

Maybe I'm missing something. If anyone one could point out a way to do this, a code example of how to do this, or a tutorial on how this is done (if it is possible) i would forever be in your debt.

Thanks in advance for any help.