MyBB Community Forums

Hi there. I was wondering how I would go about doing this. I want to make it so that when anyone tries to view the private sub-forum I have set up sees a page that says "You must be a part of 'X' to view this forum", instead of just seeing no threads inside it. Anyone know how I would do this?

You can edit the forum settings to make the forum only visible to the members in that group. All other groups won't see said forum.

I want users to know its there, I just don't want them to be able to go inside it. Like the way the "donor access" works on here.
https://forums.alliedmods.net/forumdisplay.php?f=52