Flash-mob

noun
1.
a group of people mobilized by social media to meet in a public place for the purpose of doing an unusual or entertaining activity of short duration:
The flash mob brought wide smiles to the faces of commuters waiting for their train.
2.
a group of teenagers who have contacted each other by cell phone and social media and gather in an area to trash it or to mug and beat passersby.
verb (used with object), flash mobbed, flash mobbing.
3.
to crowd around (a place or person) in a flash mob:
Dancing teenagers flash mobbed the store.
4.
to commit violence against (a place or person) in a flash mob:
The mayor imposed a curfew on all males under the age of 18 after two blocks of downtown Philadelphia were flash mobbed and severely damaged.
verb (used without object), flash mobbed, flash mobbing.
5.
to organize or take part in a flash mob.
noun
1.
a group of people coordinated by email and social media to meet to perform some predetermined action at a particular place and time and then disperse quickly
noun

a group of people who organize on the Internet and then quickly assemble in a public place, do something bizarre, and disperse; also called inexplicable mob
Examples

Organizing a flash mob involves emailing a group with instructions to show up at a certain place and time to do something and then disappear.
Word Origin

origin was event called Mob Project
Usage Note

flash-mobber, flash-mobbing n