How do you not no the game is set up that all 3 of them have to get it right or it all effects the 1 score? It may be a coop mode where they have to dance in sync.
I dont know, but it could be that.
However I also thought about this awhile ago. How would 4 players jump around in front of a TV together anyway. There would be alot of getting in each others way.
Remember this is no controllers. So you have to physically move.
I know it is only scoring one person, the guys at the Harmonix booth explained it quite thoroughly and showed how it sees three people but only one person is colored white which is the one the Kinect is actually looking at for the scoring.