When you have guests in your house you must see them off completely.
Walk to the door with them, say your goodbyes at the door and then come outside yourself onto the porch. Wait until they have got into the car, and then give them a quick wave.
You don’t have to wait until the car has left your vision, just stay on your porch until their car has started before you move to go back inside.
If there’s a torrential downpour, escort your guests with an umbrella to their car.
Whatever you do, don’t open the door, let them slip through and then shut it.