You can set up your site so that site users can register.  You can then give your registered users access to certain pages or modules that a non registered user would not see.  You can even set up security roles so that you can have categories of users that have access to different things.

Check out these videos and FAQs that will show you how to navigate the user registration and security roles features.

Adding Different Security Roles

Setting Page Permissions

Adding Users and Assigning Roles

Setting Module Permissions

Enabling User Registration

The User's Interface