This sounds like a really good idea, it would certainly be something I would use. How about combining this with a User group mod. So you could add/delete users from multiple user groups, which you could then use to control access to private forums.
To give an example, if you had a board with country based special interest areas, Each person could be added to a user group for both their country and region (e.g. In my case UK and Europe). Then those user groups could be used to grant permission to the private forums for UK and Europe. Therefore someone who wasn't added to these groups wouldn't be able to access these forums. If you added a new private "UK Sport" forum, simply giving the UK user group permission would enable all the relevant users access.
The real power of this idea is when you add a new private forum, by granting permission to an existing user group you can give access to many users in a single simple step.
Sounds more complicated than it is, I hope everyone can understand my explanation. No idea how complex thsi would be to implement, or how deeply buried into the UBB code the permissioning code is.
Cheers
Websorceror
