It's been discussed a few times how to do it, several different approaches suggested, each of them susceptible to the things I've outlined. Not to mention groups, permissions and plenty more besides.
There was a mod I saw a bit back that created multiple virtual forums under a single container install, but that's not quite the same thing, of course.
In order to really achieve it successfully, you'd need to have proper segregation of user data from other data, none of this pushing it into the members table: you'd have groups in their own table, you'd have post count and other forum dependent (as opposed to member dependent) data in another table too, so that it could be retrieved through joins rather than bulking out the members table - and segregating it means you can share parts of it in isolation rather than sharing lots and filtering out what you don't need.