Yours, Etc.: Origins and Uses of 8 Sign-Offs
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
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.
origin was event called Mob Project
flash-mobber, flash-mobbing n