Sync new clients added in civiCRM to mailchimp
There is an extension that keeps MailChimp and CiviCRM synchronized. You can find it here

This extension allows you to configure groups or smart groups in CiviCRM so that they synchronise (in either direction) with Mailchimp lists.

The extension is compatible with CiviCRM 4.7.x and 5.x.x versions (Extension compatibility with 4.7.x generally implies forward-compatibility with 5.x.x)