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