You can use ASP.NET membership and then you need to create credentials field (Or Entity) and roles for the contacts.
Read through the above link and then create Membership system as described in link below:
Walkthrough: Creating a Web Site with Membership and User Login
YOu can also use ADX Portals. Using Adxstudio's Portals product you can use the Contact entity for authentication mechanisms that Adxstudio provides forms authentication, or some other identity manager like Facebook and Twitter. It's also very simple to create Web UI for CRM entities.