Mailchimp for WP and WP-Members

You can add a checkbox in the fields tab. However, if you want that to result in some type of action outside the plugin, you would have to program that into a function and hook it to an action in the plugin.

The plugin has an action that comes after a user has successfully registered – wpmem_post_register_data. You would be able to write a function hooked to that action and add the user accordingly.

Note: there is also WP-Members support in the AutoChimp plugin and there is a MailChimp extension available to members of the premium support site rocketgeek.com