identify users by - unique ID/ unique User name

Hi,
client of ours uses an erp program to add the sites content, problem is that users arent created in nop commerce website but in the erp program including username.
he has 3 different clinics that uses the same email and when they log in to site they see same history profile data,
what is the best way to identify users by - unique ID/ unique User name - and not by email?
Thanks

Customer Settings (Advanced page) has a "'Usernames' enabled" setting:

"Check to use usernames for login/registration instead of emails. WARNING: not recommended to change in production environment."

but I think registration still requires a unique email.