Magento 2 Guest to Customer - Cynoinfotech
https://www.cynoinfotech.com/product/guest-to-customer-magento-2-extension/
This extension converts guest customers to registered customers after the checkout. it will assign account to any groups. Magento 2 guest to customer extension supports eCommerce site in converting a guest into a registered customer based on the order information. All customers like to use guest checkout neither than register or log in on the website. Because checkout as a guest customer is the fastest process and escape customers from unnecessary emails.