The idea behind the party is that people just turn up on the Circle Line on Friday and have a party. That simple. It's sort of like a flash mob for those who know what flash mobbing is (loads of people turning up "randomly" in a place where they're not normally expected). It's all very jolly and good natured but the element of "randomness" takes normal commuters by surprise.
At the last one there were pole dancers (plently of poles on tube carriages), music, booze, streamers etc etc. Anyway the timing is all kept pretty last minute so head off to the site on Friday morning if you want to find out more about exactly what time it'll be taking place.
Love the lyrics to Going Underground being used in the introduction to the site.