how to add newsletter subscription during registration?
you can achieve this either by using Mailchimp Module 

                          OR(by custom coding)


You can alter your registration form using hook_form_alter. Now in your form you can add add two new elements that is :-


A checkbox saying- Signup to receive alerts.
A text field to capture email address.


Now you can override your form on submission and pass the email captured above to mailchimp function that adds users to email lists via ajax call.

Hope this helps!!!