Explain beforehand that bumping should be gentle and bandanas can be used if you can't trust yourself to keep your eyes closed.
Everyone walks around with their eyes closed in a small space.
When you bump into someone, ask "Pruie?"
If they ask "Pruie?" back to you then they are not the scout you are looking for and you continue looking amongst the crowd.
The referee has whispered to one scout, telling him that he is the "Pruie". The pruie does not have to close their eyes and if someone bumps into them and asks "Pruie?" they do not respond. This is your clue to join onto them and open your eyes.
The fun comes when there are few scouts left to find the Pruie.