Click the customer row in the cart panel (it reads Guest Checkout by default) to open the Add Customer modal:

• Type in Click here to search by customer name, phone or email — matching customers appear as you type; click one to view details (Show Customer Details: name, created on, billing address, last checkout, phone, birthday, email, Digital Wallet balance, allergens and dislikes) and press Select, or Edit to update them.
• Guest Checkout places the order without an account.
• Create New Customer opens a form with First Name, Last Name, Email, Password, Phone, Birthday and an Add new Card button; Save creates the account and selects it.

Selecting a customer with saved cards enables Customer Card on File at payment and makes their Digital Balance available.