shabbat elevator

Question

When using an elevator that automatically goes up and down, does one need to be carefull not to interupt the beam and prevent the door from closing. I would assume if it is already closing one may not stick his hand in the way of the beam and cause the door to open, but what if you want to hold the door for someone (ie an elderly person) can you keep your hand or body in the way of the beam while the door is open to prevent it from closing?