Can either be done as a one large group or break into smaller ones. The larger the group the less likely it will be for everybody to be synchronized.
Each group nominates a leader who then proceeds to carry out a series of actions which everybody else then has to copy exactly. The person in charge might have to steadily suggest some possible movement ideas to prevent constant repetitions of obvious movements. Think along themes such as e.g. tall, small, quick, slow, a direction, etc.