Well on that occasion it was well planned and coreographed for the purpose of the advert.
There are groups who advertise on the internet though, to see how many people they can get to meet up and do such things. They are called flash mobbers and if you search on 'you tube' you will find some great flash mob stuff, such as silent raves, flash mob freezes (the best one being the Grand Central terminal Freeze in New York...fantastic work! Anyway, if you just search for flash mob on www.youtube.com you should find some great ones!