Joomla is so much more than just a powerful content management system. Here is a list of features "out of the box," but the true power of Joomla is in its extensibility.
Joomla has a registration system that allows users to configure personal options. There are nine user groups with various types of permissions on what users are allowed to access, edit, publish and administrate.
Authentication is an important part of user management and Joomla support multiple protocols, including LDAP, OpenID, and even Gmail. This allows users to use their existing account information to streamline the registration process.