I think the problem is that you've given your friends full access to your WiFi network. So they can see any shared computers, etc., and can also see your ATV via the Remote app on their iOS devices. If you can set up a separate guest network on your router and only give them access to that, then I don't think they'll be able to see your ATV anymore. They'll basically just be able to surf the web and check email. If you want to give them more access (e.g., ability to view your iTunes library or something), then there may be no way to give them that level of access while still keeping your ATV private.