To send custom membership email campaigns, you must first set up your Mailchimp integration. To learn more, go to: Set Up the Mailchimp Integration
Once you have successfully synced your account to Mailchimp, you need to create a segment for clients with active memberships.
To do this, log in your Mailchimp account and go to: Audience > Segments
In the Segments page, select Create Segment
Filter your list as shown below and select Preview Segment
On the next page, select Save Segment. Name your segment as Active Memberships, for example and select Save.
When creating your email campaigns, you can now choose the Active Memberships segment on your Audience list.