Answer: flashmob

AnswerCrossword Clue
FLASHMOB

Spontaneous assembly often organized online

FLASHMOB

Virtually organized crowd

FLASHMOB

Apparently spontaneous public gathering

FLASHMOB
Spontaneous public gathering
FLASHMOB
Seemingly spontaneous public assembly
FLASHMOBGroup behind a public stunt
FLASHMOBSeemingly spontaneous gathering