The Ninja Forms MailChimp Opt-ins extension allows you to quickly add newsletter opt-in option for any Ninja Form.
Ninja Forms 3.x add-on features
Newsletter subscriptions are handled as Ninja Forms Actions.
Ninja Forms 2.9.x add-on features
Invisible form field (‘None’) for simple forms where you only collecting first name and email address.
Checkbox field for forms where you would like to give an option for your site visitors to opt-in.
Dropdown field which would let your websites visitors to pick one of your newsletters from the list.
MailChimp Merge Tags
Ninja Forms MailChimp Opt-ins supports custom MailChimp merge tags. Merge tags could be used to populate personalized or dynamic content in your newsletter campaigns, response emails, and automation workflows. If you not yet familiar with it, check out getting started guide on mailchimp.com.