In 'Usergroup Permission', there's no option to set access for a specific forum. It only provides general access permissions to all forums. To effectively sell access to a specific forum, users need a glimpse of its content. For example, they should be able to see the thread titles but be redirected to the forum access product in the store when they click on them. I understand this feature might not be currently available in your plugin, but it would be incredibly useful if added. Partially restricting access makes people curious about what's inside. This curiosity can motivate them to purchase forum access.
Of course, I can market this separately, but I also want to demonstrate the forum's activity with each new message. I live in Turkey, and people here value activity. To show that the forum is lively, you could consider making access a bit more flexible, similar to what Theme House does in its Monetize plugin. Currently, a new member can't see the forum at all, which doesn't encourage them to purchase access.