This can be achieved with the help of the RSMail!-Joomla! registration integration available for our newsletter component. This integration will add all new users that are created on your site to your desired newsletter list. You can read more about this topic here.
Since RSMembership! uses default Joomla! functions in order to create users, when an user is added through the membership subscription process, the above mentioned integration will detect it as well and add it to your newsletter list.