5
I think you can't do it in the current offical XOOPS release (2.0.7). You can, however, set a forum readable by individual users (assign one by one), instead of a group.
Note that in module newBB 2.0, a very sophisticated access control scheme is designed. However, it's still in Beta stage:
http://dev.xoops.org/modules/xfmod/project/?newbb