Dropped ball is a method of restarting play if play was stopped by the referees for reasons other than infringing of the Laws of the Game. Instances are like serious or bleeding injuries, disturbances by outside elements (spectator, coach, etc.) or accidents (cross bar or goal post broke, unfavorable weather conditions).
The referee drops the ball at the place where it was when play was stopped. The ball may not be played until it touches the ground and if it leaves the field directly, without touching any player, the ball shall be dropped again. Any player may challenge for the ball and no minimum or maximum number of player required to take part in a dropped ball. The referee may drop the ball even if only a player is there. The referee cannot decide who may or may not take part in a dropped ball. Is the whistle needed in a dropped ball?
Often referee misunderstand with guidelines given by FIFA: Whistle is needed to restart play after it have been stopped due to the issue of cards, injury or substitution. The whistle here is not to stop the game but to hold the game for the reasons mentioned. You don't stop play for substitution, do you? It is because you stopped or hold the game you have to get every one's attention again by using the whistle. So if you stopped play for a serious injury or bleeding injury, you need to dropped the ball but no whistling is needed. The ball is in play the moment it touches the ground. Reminder: Referees should not stop play for minor injury. Cramp is not a serous injury so game need not be stopped.