Some ideas on how to make audience participation necessary or a free adaptation of the Tinkerbell Effect.

A direct address to the spectator by the actor.
Give a precise role to the spectator.
Tell the audience that the show won't start until everyone arrives. Do not specify how many people are expected. Wait a little. Let one last spector/complice in. Start the show. 
Film the show in live instagram. Let the internet users decide what to do next.