Volleyball: sand or indoor, but with people

It's the sport here that depends most on gathering an exact number and needs the least infrastructure to start. A net on the beach and eight people is the whole of it, and the eight people are the hard part.

What defines a game

Sand and indoor are two sports
Four-a-side on sand and six-a-side indoors share the rules and not much else. Anyone coming from one notices the other on the first point, so it's said up front.
The number is exact
One short and it doesn't play the same; one over and somebody is watching. Publishing the places missing is what gets the game started with the right number.