membership and groups

I want to be able to protect a page based on whether or not a person is a member of a certain group. But I don't want people to have to do anything other than join a group to get access. So not a new membership level. Is this possible?

Maybe a check box on the page editor that says "Person needs to be a member of x group to view this"

Right now I am just looking for a hack that I could add to the page code (page template) I am using to do this.