The great thing about circuit training is; you need only a few things to get started; space, maybe a bench and a timer.
Intervals takes care of the last item for you. Set the duration of activity, then the time you want to rest between activities, then hit start! Intervals will let you know when to go and when to stop with simple visual and audio cues.
Got a big group? Just plug it into a dock, turn up the volume, and turn up the tunes.