Actually I don't get the point what that has to do with ISPConfig?
What do you mean by "using the internet" ?
If you need onyl http / ftp I would do sth. like a squid proxy that authetnicates against an ldap directory.
Another possibility would be to have a firewall that opens special ports, destinations etc. (maybe defined in an ldap tree) by an authenticated user
But that depends on you needs, as well as the hard and software you have...