Because of the way Revoke works in xenForo you shouldn't use it to restrict a private forum. Instead you should use a special feature in xenForo called Private node. You will see the Private node checkbox when editing the permissions for a specific node. This basically inverts the permissions so that you can specify Allowed groups instead of Revoked groups. This is actually better for group management if you add more groups later.
Admin CP -> Users -> Node Permissions -> [click Permissions for a forum] -> Private node
I've been using XF now for almost three years with great success. I am trying to set up a private subforum that's invite only and invisible to all but those specifically invited.
I've created a separate permission group that will allow access to this subforum. The problem that I'm having is connecting this permission group to the actual subforum.
How do I get this permission group into this subforum? I created a new category for "private forums" and am about to create the first subforum under it. I know how to make them private thanks to Jake's post above.