Do it with 5 plus or minus 2 people, not more. Constraint fosters creativity, not the other way round. Park your ego and attachment to your ideas at the door. Individual work beats groupthink and brainstorming. Dictatorship is better than democracy when it comes to product design. You can prototype anything in a day if your constraints are tight. Bravely show your work and take the critique. There are no failures, a rejected prototype tells you at the very least what not to do.