Mod Description: Allows you to set up a forum section were users got to pay a one time fee to Access it

Plugin was created and designed by Nickman
I converted it and i had to remove the logs script which was linking to the old credit system

Link to Mods DB: Get It

what about making it to pay each month or set amount of time to pay fee to keep yourself in that group? This mod has been created by Nickman for My Credit so you may give a try?

i'll talk to nickman and ask if he wants to do it as a future release

That's what Group subscriptions does :)