I know it was asked a long time ago. But just a tip for others who see this.
You can use global $wpdb in any wordpress file to access the pre-defined class. If you know sql you can create a custom query to find active or inactive memberships from one of the memberpress tables.
For reaccuring memberships: $wpdb->prefix.’mepr_subscriptions’
For non-reaccuring: The table is $wpdb->prefix.’mepr_transactions’. It has all the past and current transactions for every user, so you would have to ensure your query only grabs the most current one.
You should consider staging the site and using phpmyadmin to set up the query.
goodluck.