Mailchimp addon for UsersWP.
This addons lets the user subscribe to your newletters via Mailchimp during registration.
When activated this addon creates a “Subscribe” checkbox in register form.
Keep in mind that you must have already created Mailchimp API key and Secret in mailchimp official website and then saved those details in UsersWP > Addons > MailChimp settings page.
You can use the shortcode [uwp_mailchimp] to display the mailchimp subscribe form.