WP User Frontend Mailchimp add-on enables you to add your newly registered users to MailChimp's subscriber list automatically. It is really easy to configure and use. You just need to get the API key from MailChimp and enter in the add-ons settings. The add-on will automatically fetch the lists from MailChimp and show in the settings page.
You can choose the list to add newly registered users from the dropdown. Like you can make a registration form for customers and one registration form for suppliers and both can be subscribed to different or same mailing list.