You don't need any posts in the forum visible.
You just need a public post somewhere explaining how to access the opt-in groups.
One of the "Read Me" stickies is the logical place.
And you don't have to lock/set permissions for all forums. Just ones that would be considered "narrow" or "specific"
This forum is a good example. So would the game-specific subforums in LFG.
If I'm not looking for or following a D&D game, then I can choose to hide the D&D forum.
Same for Running Campaigns.
I see that phpBB has a groups feature, found here: ucp.php?i=groups&mode=membership
Just set some groups to "Open Membership" and that should do it.
If you want, make a test forum, lock it to members of "TestGroup" and make it open membership. Then let some people know how to join the group, and voila.