Users connected to clients?

I finally got my ecommerce set up.

But, I am uncertain as how to create a user that is connected to a client account.

I am using the ecommerce as an order desk for existing B2B clients.

I have my client account created

I also added a new user, and gave them website user status.

But, I am still at a loss as how to link a user to a client account?

Or, do I need to use the name/email of one of the contacts listed under that client account?

Or can I take a current contact for a client and make a user account for/from that contact?

Thanks for any ideas on how to do this.