The players are divided into equal teams and lined up on the serving area of the same half of the game, with each team having a passer and a catcher on the opposite side.
The first player of a team serves.
After passing and catching, catcher throws the ball to the next server (or becomes a server himself).
Catch joins server, passer becomes catcher
etc
If emphasis is on service: wrong serve means again.
If emphasis is on passing: wrong pass means stay put (again).
If the teams are small, it is nice to have everyone serve two or three times.