Software Engineering: Organize different kind of users

Alright people,

In the last couple of days I've been thinking how to implement this properly, and I'd like to know what your approach would be to implement the following scenario:

• I'm doing an eCommerce platform, and we have many kinds of "entities". Entities are not necessarily users that have credentials and can login in our platform, and everything should be fairly decoupled from everything else.

• Real life scenario: We have customers, employees and suppliers, all of these can or not be a User, as in, they may or may not have login credentials. I can attach a contact (address info) to any of these models, or an invoice... The idea is that we may have customers that don't actually have credentials that we issue invoices to, or create an order to a user that doesn't actually login... Same for invoicing, I want to be able to invoice a customer, as well as a supplier.

The problem that I'm having is that these 3 types of user are more or less the same, they can have orders/invoices/contacts/whatever attached to them, but they aren't necessarily a user. In fact, credentials must be something that we should be able to attach to any of those models, if we want, so a supplier can have a login to access whatever they need.

How would you design this?