Depends on how far into the crisis we are. I know if they could knock, they could at least have the decency to yell (assuming they knew it was me in there and knew me) "Hey, it's me, [friend name/mom/dad]" lol.
I wish I could say that I probably wouldn't open it, but fundamentally I'm too good sometimes. I would probably open the door regardless.
My husband says that it would depend on how they knew someone was wherever we are, and how early on it was. The more early on, the more likely he'd let someone in, because they're less likely to be sharks. He said the first month of the crisis is going to be the best time to find people. Plus, if they've lived through the first month, they're probably good assets.