If your employer tells you that you have to do this, you have to--an employer has the right to determine the parameters of a job, which can include requiring a therapist to transport, or arrange for the the transportation of, clients. There is nothing inherently illegal about a therapist or an employee transporting clients; therefore, it is entirely up to the employer whether you need to do this or not. If you do have to transport them in your own car, however, you may need to contact your insurer and get different or additional insurance to make sure you are covered for an accident during a business-related or commercial trip.