MyBB Community Forums

When I set the permission so that guest's can't view, they can't see it all together. How do I make it so that it gives them a message that says that they must register to view the forum, when they open it?

ACP > Forums & Posts > choose "forum" > Options > Forum Permissions in "forum" > choose "Group" > Set Custom Permissions

I can set them to either not view the forum at all, meaning they won't see it, or, if I set the custom permissions to view, but not view threads, then it shows that there aren't any threads.