XF 2.1 Allowing Access To A Hidden Forum


I am trying to set up an option for people to be able to opt in to a free forum thats hidden from view. This is done so people dont have to see the new posts, etc if they dont want to. Ive set this up on XF1 before but I cant seem to get it to work right on XF2.

Ive created the forum. Ive created the usergroup to give access to the forum. I know I need a custom user field so it can be selected but Im not sure how to tie it all together from there.

Can someone point me in the right direction?