Answer: flashmob

AnswerCrossword Clue
FLASHMOBSpontaneous assembly often organized online
FLASHMOBVirtually organized crowd
FLASHMOBApparently spontaneous public gathering
FLASHMOBSpontaneous public gathering
FLASHMOBSeemingly spontaneous public assembly
FLASHMOBGroup behind a public stunt
FLASHMOBSeemingly spontaneous gathering