Can I knock on someone's door?

Yes. Point at a locked door that isn't yours and you'll see a Knock button next to Open. Tap it and everyone nearby hears the knock — including whoever is inside.

The button only appears on doors you have no access to. Your own doors, and doors on a base where you're authorized in the cupboard, don't get it — you can just open those.

While the knock is still playing you can't knock again on the same door.

What you do with it is up to you: check whether the neighbours are home, start a conversation instead of a fight, or make yourself thoroughly unwelcome.