Magento provides you with guest checkout functionality so that you can create a great customer experience without forced checkout. However, you can still gather customer information and convert guests to customers in Magento 2. 

You just need the Magento 2 Convert Guest to Customer extension that allows you to do it both manually and automatically.

How to Convert Guest to Customer in Magento Manually?

The Guest to Customer Extension adds the Convert Guest to Customer button to Order, Invoice, Shipment, and Credit Memo pages. So to convert guest checkout customers to registered customers in Magento, you need to go to any of these pages and press a corresponding button.

Magento 2 Convert Guest to Customer,Order View

You can also convert guest orders to customers directly from the orders grid. 

Magento 2 Convert Guest to Customer, Orders

How to Convert Guest to Customer in Magento Automatically?

To enable automatic conversion, go to Stores > Configuration > Magefan Extensions > Convert Guest to Customer Extension and enable the Automatically Convert Guest to Customer option.

Additionally, you can choose order statuses to Automatically Convert Guest to Customer on certain Order Status.

Magento 2 Guest to Customer Configuration

This way all your guest orders will be automatically converted to customers after checkout. Even if a customer already has an account, their guest order will be automatically assigned to their account based on the email address.

In case you want customers to do this on their own, you can allow customers to assign guest orders to their accounts. And if you have some old guest orders, his module will convert all old guest orders to customers