Users and robots are handled separately.
Users: the easiest way to show some content only to registered users is by putting it in specific categories, then only granting read permissions for those categories to registered users. People will have to be logged in to access them.
Robots: by default, automated entities like Google web spiders can access anything a non-logged-in user can access. They can't log into your site, and therefore can't see anything in a "registered users only" category.
You can further dictate what a bot should(n't) access in the robots.txt file and/or robots meta tags.
Also, robots see all of a document or nothing. You can't hide parts of a doc from them.
Hope that helps!