Not sure I fully understand your question! But if you are asking about private members only forums (membership by invitation only) simply create the forum, set the 'Visible, read, post, and reply" permissions for registered and anonymous users too NO; assign a group leader and proceed.

Anonymous and registered users will not see it and the only ones who can see read or post will be the members physically added by the admin and or the assigned group leaders,

You can also make it so the forum is visible but not readable so your users know its there (add a comment in the description telling the users who to request membership through). You might also want to include some htaccess files and or robots text files to further discourage prying eyes.