Locking forums for guests?

[Chruser]

I know there's a possibility to lock up the mainpage of the forums, so the guests HAVE to register before being able to view the forums at all. Also, you can "hide" all the forums for guests, which will make the forums look empty.

My question is, how do you let users access the "mainpage" of the forums, so they can see the cathegories, while they would have to sign up to actually see what's in a cathegory?
I think this could be done with some very simple coding, maybe even some option that I missed, but it would be great if you could help me (I know more people looking for a feature like this).