After approaching the car, place the hand of your guiding arm on the door handle to allow the person to follow down your arm and locate the handle. At this point, the person can open the door and seat himself or herself safely into the Car.
Depending on how unfamiliar the situation is, it may be helpful to let the person know the direction the car is pointing and which door they will be using. For instance, “The vehicle is pointing to your left and you will be riding behind the driver.”
Once the person is seated in the car, they can close the door on their own, or if you prefer to do so, be sure to let them know you will be doing so.