Mailchimp User Sync
With Mailchimp Sync you can keep your WordPress user base in sync with any of your Mailchimp lists.
After installing Mailchimp Sync, it will automatically start listening to changes in your WordPress user base. When someone updates their email address, the plugin will make sure the change is reflected on your Mailchimp list as well.
You can map additional user fields to a Mailchimp list field to keep those fields in sync too.
For existing users, the plugin comes with a live wizard to synchronise all existing users. If you have a very large user base, the plugin offers WP CLI commands to speed up the process.
Another option is to also synchronize changes made in Mailchimp back to WordPress. For this the plugin will attempt to create a webhook in your Mailchimp account.
Mailchimp User Sync is a premium feature as of January 2020.