Every gaming group has a social contract, either implied or explicit. What's yours?
My group runs on Roll20 on most Tuesdays from 7:30pm and no later than 10:30pm. I send out an email on the Monday before and the players reply "I'm In!" if they can attend. If they can't then we appreciate a "sorry!" email (we are British after all) but no answer means no. The bare minimum is three players to play. Fewer than that and it's automatically cancelled. I'll always put out an email confirming that's the case.
What do you do? How do you organise yourselves?