New customers that purchase something in Momence are automatically added to your Mailchimp audience. In other words, if you add a new customer to momence, and they haven't made a purchase, this will not add them to Mailchimp.
Every time someone makes a purchase in your Momence account Momence checks with Mailchimp to see if the customer already exist in your Mailchimp account. If they don't, they're added.
To integrate Mailchimp:
click Apps and Integrations > Integrations > Mailchimp > then sign in to your mailchimp account in a separate window.
Momence will need the following info from your Mailchimp account to set up the integration:
Api Key
Audience Id
Server prefix
The video below will show you where in the mailchimp UI to find this info.