Multiple database prefixes all with the same user login info?

What I really need is sub forums. These aren't available right? Do any of you know of a free bulliten board that offers sub forums? I'd like to use punBB because of it's simplicity and small footprint... but I just don't think it's gonna work.

To try and get around the problem I've thought about using  multipal forums, such as:

/forums/business/
/forums/engineering/
/forums/nursing/
...and so on...

I htought this would work very well... but then I though of the user information. Each user would have to register in each forum.

What should I do?