Unfortunately this plugin doesn't work and the premium version that I was sold, doesn't either. Whether a user checks out via WooCommerce as a Guest or creates an account, we have received nothing but errors and NO users are auto-created within Mailchimp. The error we constantly receive for the past 2 months is:
MAILCHIMP ERROR: (250) MMERGE3 must be provided – Please enter a value
Check the right rail of any order under "Order Notes". A private "Order Note" is created for errors related to this plugin and Mailchimp.
It simply doesn't work. The issue seems not to be related to Guest vs full user account checkout. All plugins and code is up to date.