Magento 2 Guest to Customer - Cynoinfotech
https://www.cynoinfotech.com/product/guest-to-customer-magento-2-extension/
Magento 2 Guest to Customer Extension developed by Cynoinfotech helps to add a button in "Sales Order View Page" which will convert the guest into a customer based on the guest order information including email and password details. A guest customer can login as a customer using these details. The Guest to Customer Magento 2 Extension allows you to assign the appropriate customer group from the configuration to convert guests to registered customers. You can save billing information to avoid entering data for future orders.