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 guest checkout customers to registered customers in Magento 2. 

How to achieve that?

With Magento 2 Convert Guest to Customer extension that allows you to do it both manually and automatically.

To convert guest to customer in Magento 2:

1. Go to Admin Panel  > Sales  > Order.

2. Select a guest order you want to convert to a customer and press Convert Guest to Customer button.

Magento 2 Convert Guest to Customer, Orders

3. Convert Guest to Customer from the order details page too if you need.

However, if you don't want to regularly update guest orders and manually convert them to customers you should consider automatic conversion.

Magento 2 Convert Guest to Customer,Order View

4. Go to Stores > Configuration > Magefan Extensions > Convert Guest to Customer Extension and enable the Automatically Convert Guest to Customer option.

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.

Magento 2 Guest to Customer General Configuration

Magento 2 Guest to Customer extension provides you two different options to convert guest orders on your store into registered customers.

However, what about those old guest orders you have on your website?

This module covers it too and allows you to convert all old guest orders to customers