Assuming you don't want all members in the regular user group access.
For stock code, groups would be the way to go.
Say add a forum called "PMforum" and a group called "PMgroup".
Set permissions on "PMforums" to only be visible to "PMgroups" and of course admin.
Then add the members to "PMgroups".
Or they could be added auto mode if you set a post count to the group.
