This plugin allows you to take one-time or recurring payments in exchange for access to a group. Effectively, that allows you to put categories behind a paywall and opens up a lot of possibilities that allow the monetization of your forum.
If you want to get a feel for how this works, you can see a demo setup here. Use these credit card numbers on checkout:
#: 4111 1111 1111 1111
Date: Anything in the future