UsersWP MailChimp Addon GPL Overview:
The Mailchimp addon for UsersWP lets users subscribe to your newsletter via Mailchimp during registration.
Mailchimp add-on for UsersWP settings page
After activating the add-on, you will find its settings page at:
UsersWP > Addons > Mailchimp
On this page, you have to add a Mailchimp API Key. You can create one through the official Mailchimp.com website. You can find the documentation about it here.
You’ll also have to add a Mailchimp Audience ID. The Audience ID determines to which lists your users will subscribe. Documentation about this can be found here.