The court is there; the people aren't

Public courts are empty half the afternoon and packed the other half, and you never know which one you'll get. Turning up with a ball and waiting to see who comes down is the traditional method, and it works about as well as it sounds.

How a pick-up game comes together

3x3 and 5x5 are not the same plan
One needs six people and the other needs ten, and the game changes entirely. It's said up front because it decides how many have to be gathered.
Half an hour's notice isn't enough
Gathering six or ten takes warning. A game posted a day ahead fills; the same game at six for seven does not.