Magento 2 Guest to Customer - Cynoinfotech
https://www.cynoinfotech.com/product/guest-to-customer-magento-2-extension/
Magento 2 Guest to Customer is an extension that adds a "Convert Guest to Customer" button in the Magento 2 admin panel to the Order, Invoice, Shipment and Credit Memo pages. It allows you to automatically create customers based on guest order information. Magento 2 Guest to Customer Extension is the solution you need. With just a few clicks, this extension will convert any guest order into a registered customer account, transferring all order and customer information automatically. Not only that, but customers will also receive instructions on how to access their new accounts.