According to Business Insider, 28% of US online shoppers abandon their cart because they don’t want to register/create account just to make a purchase.
ASOS’s approach removed the account creation step on the checkout login page and it reduced their abandonment rate by 50%.
Offer the guest checkout as a prominent option. This means that it could be selected by default, and visually attract more attention than the “Sign in” option.
The same email address should work with multiple guest checkouts, to allow the same person to make more than one guest purchase with their email address.
Offer account creation after checkout, in the Thank You page.