Anonymous users can be converted to named, authenticated users through the Customer Portal, or portal for short. A portal allows you to extend Force.com to your partners and customers without requiring full user accounts for each of them. It is tightly integrated with Force.com Sites.
Enable portal integration by clicking the Login Settings button on the Site Details page. In the Login Settings section, click the Edit button and select an existing portal from the drop-down list, and then click the Save button. Figure 7.6 shows a site enabled to log in to the portal named Customer Portal.