Split the group into pairs and organise them so they are standing opposite each other. One person makes a movement, the other tries to mirror them as well as they can, they then continue this for the time allocated (say a minute). Swap roles. When both have played both roles, they can try to coordinate movements with each other, so that both become player and mirror at the same time. This works best with slow movements and needs a lot of concentration.
You can also use this to help practice certain movements for sports e.g. passing the ball, en-guarde for Fencing, kicks for Karate etc (you get the idea). Just have more space available if being used for these purposes.