I figure you could have forums specific to one of the sites by creating a new user group. When you edit the config.inc.php file for that site, you can have them go in as not regular users, but as members of the new user group. Then set the permissions on forums you only want them to see so that only that group of people can see the forum.
Since each site has it's own separate config.inc.php, you could have all sorts off user groups set-up to see forums assosciated with a certain site. I hope that makes sense. That's what I would do at least!
