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