Once you have setup the api key to MailChimp in your portal you will be able to add a client to a list you have setup in Mailchimp. To get started you can select a client from the list. Once the client loads you can scroll down to the Activities section where you will see an Add To text and icon. Once you click that you will see a Opens Settings link underneath the MailChimp logo.

Once Open Settings is clicked it will popup a box which will show you a list of all mailing lists you have setup in MailChimp. You can select one and click Save. Once you do this the clients will be added to Mailchimp and if you have a workflow welcome email setup it would send that email.

It takes a minute or 2 for Mailchimp to send the email. You can check back and once you click Open Settings again your will see the Campaigns tab. On the campaigns tab it will show you all of the campaigns sent by Mailchimp. With their paid plan you can setup Automation Journeys that will send email based on conditions and actions.