Yes. If the email you enter already has a FleetKraft login with another company, the driver is added to your company with that same login. Their password does not change, no temporary password is shown, and FleetKraft emails them that they have been added.
Each company sees only its own jobs, shifts, pay and messages for that driver, and the driver can be on duty for one company at a time. Their name and phone number belong to their own login, so you cannot change them from the driver's page while they also work for another company.