The User module (1.3) for ExpressionEngine allows you to bring standard EE member functions into regular EE templates. Though not all functions are currently available, most of the primary functions are.

The module allows you to create the various pages and forms to let your members maintain their profiles without having to use the native EE member templates and functionality. Freedom is a good thing.

Additionally, User includes the ability to invite people to join your website. You can generate an invitation key that when used, allows someone to join a specific member group that you have designated.