I have seen this one other site where you can not view something until you have signed up to the site/replied to the thread.
Because like TheDonBro service Guest's can still view the thread and then just wait till he has posted his gamertag and then join his game, Which then mean's they have no reason to join up to the site.
phpBB • [ABD] Easy Hide BBCode (Reply-to-see functionality)
Like this one 🙂