[Zope] user management design

[Panna Cotta]

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.