What’s the proper etiquette?

Let them open it.

Guests should not open the door when someone knocks. You don't know if the owner of the house knows this person, or if they would normally not answer the door.

I'm not the butler. They can answer their own door.

let them get it. never know when a Jehovah Witness will knock

Tell em if they didn't notice

Tell them if they tell you to open it because it should be x anyone other than that you close the door and tell them it's not... Do they want to let them in. They then decide whether to go or tell you to fob them off.

It’s their house, they open the door unless they tell me to do so.

Neither unless they ask or don’t hear

Let them open their own door

Tell them. They should answer.

Their house, their problem