You must first enable the Checkout feature under Configuration > General to access this functionality. You can enable anonymous checkout mode to speed up the checkout process for customers by selecting the Enable guest checkout option from the Configuration > Checkout menu. The Storefront will automatically create a new guest account for the anonymous shopper upon placing order bypassing the standard login and registration forms in a normal checkout process. In guest mode, the customer will not be able to login to their newly created account unless you explicitly provide the login and password to the customer.
Reuse guest account
It's important to understand the Storefront creates a new account with the customer's email address captured during checkout. By default, your site allows multiple accounts with the same email address. If, however, you have explicitly enabled the Requires unique email address under the persona bar Settings > Security page, the Storefront will not be able to create the guest account if the email has already been used before. In such cases, you must also enable the Reuse guest account under the Configuration > Checkout settings.
If a customer is checking out as a guest (i.e. the customer is not logged in) and you enabled Reuse guest account, the system will try to match the customer entered billing email with an existing account on your site having the same email address on file.
This feature is generally desirable for customers since it will group all his orders together and reduce duplicate accounts in your system. It does, however, present a potential risk for guest customers to accidentally or intentionally place orders on behalf of other customers simply by entering the same email address. For example, if your marketing entitles a known customer to a special discount, the guest customer may be able to piggyback the same discount by guessing the email address. If you have the Update user profile enabled under Configuration > Checkout settings, the guest customer can potentially override some of the other customer's profile information.
For security reasons, this feature is disabled by default. You will have to decide if the the benefit of this feature outweighs the security of the site before enabling it. Please note, the guest customer will still not be able to access any customer account information since he does not have his password. The guest customer cannot reset the account password either since all email communication will still be sent to the owner of the email account.