How do I add members to an audience

Once you have created an audience to Salesforce, there are a couple ways to add members to it.

Data wizard: Most people find this the best option to bulk subscribe and update members. You just need a Salesforce list view, report, campaign or SOQL query. That becomes the import source and the data wizard does the rest, including field mapping, setting tags or groups, and running on a schedule.
Invocable actions: We offer some actions that can be called from a record-triggered flow or apex trigger. The can subscribe, unsubscribe, resubscribe some members, and manage tags.
Manually on the Memberships component: This can be placed on a Salesforce record and allow users to subscribe or unsubscribe one or more emails from the record on one or more lists.

Note:

Field mappings do not subscribe members. They only update the mappings on existing members, in the direction specified..
Syncing data Salesforce to Mailchimp is controlled by one of the above methods. Syncing data from Mailchimp to Salesforce is handled entirely by the Primary Sync, up to hourly