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 than 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 a guest and simply press Convert Guest to Customer button.
Convert Guest to Customer button is also available on the order details page in the Magento admin panel.