A computer tells them when and where to give out prizes, it doesn't matter how many are in your group unless you happen to be at the tale end of them handing out fastpasses, lanyards, or mickey ears and they run out (they give out 100 to 250 at a time). For bigger prizes the 'pick' one person to win, and the family/friends may be able to share in that. If it is a castle stay they can only fit so many people in the room (not sure the number), but the chances of winning that are very very small.