Create a forum node that is not available to your normal users, you can also simply just make it private if you want. Then create a new usergroup which has whatever special permissions you desire. Then in your forum node permissions for this special forum you can make sure that your new has access to post, etc.
XenForo has a payment option available, so you can set up what are known as user upgrades. Simply create a new paid upgrade that adds users to your newly created usergroup. That's pretty much it.