The main difference between registered and non-registered users is
usually that the former can log in and see their orders whereas the latter
So you can store them just alike, only that you need a pseudo name and
no password for non-registered users. That is to say: 1. make the password
nullable in your table, 2. create a UUID for the user name whenever someone
orders without registering.
(Maybe you will have to make more fields nullable that are obligatory
for registered users but not for non-registered ones.)
EDIT: Of course you can also keep the password not nullable and generate
some password along with the user name that will never be used.