Everyone may start whenever they feel like it
None of the participants know in advance what is going to happen nor can they anticipate the situation. Some of them may have expected something completely different, hoping for a sports day or a trip to a theme park. Other may show a rigid attitude: 'I wasn't hired for this, was I..?' and need more time to get acclimatized. Allow them some time to adjust; the supervisors will frequently encourage them to join in anyway and be loyal members of the group.