This documentation has some good information in it if you haven't already read it.
Essentially, there isn't a direct map/relationship from a user to a contact. The relationship is via a membership mapping table. This table defines a type of member (user in your case) and the users id and links it to a contact.
So that being said, when a user:
- Register on the website
- Log in under a user account
- Fill in customer data while making a purchase
- Subscribe to a newsletter
The link between a user and a contact is created.