Background

I have a joomla based application. My customers sign up and they register as a user on the site. My customers (primary customers) then have their own space on the site that they can then setup their own customers (secondary customer).

Question/Problem

The problem I am having is that I need to tag each secondary customer to a primary customer. I thought about just creating a new component and having a separate table that includes all the secondary customers. The problem is that I then lose out on all the authentication, session handling and login/logout that the core joomla _users component offers. I then thought about just having all the users in the core _users table and add the primary customer associated with each secondary customer to a field in a profile plugin. This would work for the most part, but this means that primary customers cannot create a secondary customer with a user name that already exists in the _users table. I didn't think this would be an issue, but several of my primary customers (currently only test users) have been confused by the site telling them a username is not available, since they can only see the names of their own secondary customers.

Any ideas on some architectural changes I could make to solve this?