[Zope] user management design

Hello, I've got a question concerning user-management. I've build a forum where you have to register before using. In this forum privileged users should be able to add private areas and to invite their friends to talk with them. Therefore I need a role called member for taking part in the general, private forum and the roles vip-member and friend of vip. This are 3 roles. My first concept has one big problem: How do I prevent priviliged users to spy out private areas of other priviliged users? Thanks.