Does a Tracker project belong to one account or one per user?

Hi!
Thanks for a great product that me and our team have begun to explore and use from today.

We're not really getting the "Account" concept though. I started two accounts ("Work", "Private") and a Project for our agile project at work. The project was linked to the "Work" account. I invited a work collegue to participate in the project (as an owner him too).
He created his own account. Now, when he checks the project in the settings page, is says the project belongs to my "Work" account.
I thought that the accounts always belonged to the individual user, and that all project members needed to create their own accounts into which they could hook up to the common project.

How is this intended to work? Should we create one account for the company, and add the companys different project in this? Or should all participants create their own accounts each pointing to the common project?

I work as a consultant, and when I finish my work at this customer, they would not want me to be able to see the project at all, how could this be accomplished related to accounts/projects?

With best regards, and looking forward to an answer,

Alejandro Coronel, Sweden