Converting guests to customers in magento 2 after the checkout you have the opportunity to create a great customer experience and gather more customer information that forcing them to register.

So, in order to convert guest to a customer in magento 2 follow these steps:

1. Go to Admin Panel  > Sales  > Order.

2. Choose an order what was placed by guest and simply press Convert Guest to Customer button.

 

Magento 2 Convert Guest to Customer, Orders

Convert Guest to Customer button is also available on order details page in the Magento admin panel.

Magento 2 Convert Guest to Customer,Order View