We have a plugin for the default joomla registration form. If you turn it off it will show the newsletter fields during the registration on your site. If the user decides to subscribe to your newsletter he will be added to mailchimp.

The only stupid part right now is that the user needs to enter his names twice. But in the next version we'll add fields matching so if the user has already entered some data in a joomla field we would be able to use it and send it to mailchimp..

Regards,
Daniel