You use a user a promotion. On registering everyone has registered as primary group.
You make a user gorpo promotion (Admin Control Panel > Groups and Permissions > Promotions) that has criteria (= Apply this promotion while...) based on number of posts, and gives them a specific secondary group (e.g. group 2)
Then in that forum permissions you make it so the registered users have view node = no, but that group 2 have view node = yes