Integration with Registration?

I’d like to add the newsletter subscription (with groups) to my custom registration page. So, only a question like “Do want to sign up for regular updates?” (and then a choice of 2 groups).

It’s probably not so difficult: on clicking the ‘Register’ button mapping the Name and Email address that are already filled in on the registration form to the Subscribers list (plus of course the group chosen), but I’m not sure exactly how to do that.