wpCJ Chimp allows you to automatically add a new registered user to your MailChimp list once they registered with your blog depending on his/her selec
The easiest way to install it is using the WordPress build-in installation feature:
If you prefer to use the old fashion way then:
Once you have activate it, a new option is added to your WordPress Settings module named wpCJ Chimp.
Click on the build-in Help link at the WordPress header for configuration and usage.