If any of you were ever in The Red Lounge (and if you don't know what it was, you weren't there), may I respectfully point you in the direction of oldbaldchris.
Flashmobs occur when people arrange via email/mobile phone/etc to all meet in a particular place at a particular time - said place frequently not being specified until extremely close to the time.
This is the one that occurred yesterday in central Edinburgh. With pillows.