Decide who is going to be the 'project manager'. This person will be responsible for ...
Overseeing the task.
Ensuring that everyone has a 'job' to do, taking into account the skillset of the individual members of the team.
Ensuring that everything is in place to allow successful completion of the task.
They should have good people skills, good organisational skills, and a genuine interest in seeing the task completed to a good standard. They should be someone who is prepared to take on board the ideas and suggestions of other members of the team, but also to take control when things are going wrong.
They must NOT be someone who has a 'my way or the highway' approach to team working.
This person will not JUST project manage the task, but will also have a role or roles within the planning and preparation of the activity.