Making an ATS with Dashboards, forms and user roles.

I am trying to create a dashboard with three user roles: An admin, a seller and a client.
The client makes an order, it goes through the admin, he authorizes it, and it goes through the seller. The seller confirms and approves it and it follows the same route back.

But both the client and the seller have to be registered through signing up pages or forms. Before they are added to the system, the admin has to approve it and assign them a dashboard of their own.

Does anyone know how I can go around this?