Adding view controls to a page

[Merriweather]

I have some parts of my forum that are currently open to anyone, guest or registered. I want to add controls to those pages so I can set each usergroup's permissions whether they can view the page or not -- basically, I want the "You are not logged in or you do not have permission" message to come up if a group I deny permission to tries to view that area of the forums.

I realize there's a bit of work involved here, but I wondered if someone could be kind enough to help me muddle through adding this control to a part of my forum?