This add-on plugin will provide MailChimp integration for Restrict Content Pro that will allow users to add themselves to your MailChimp email list when they register. It works for both new registrations and upgrades.
The plugin requires that you have Restrict Content Pro installed, and that you have a MailChimp account (free).
Note: a dramatically improved Pro version of this plugin is available from Dan Griffiths of Section214. The pro version includes level-specific subscriptions, eCommerce tracking, automatic subscriptions, status tracking, and more.