This is hysterical. It's called "Improv Everywhere" and it's been going on since 2001. The group does stuff like:
Have everyone go into Best Buy dressed in khaki pants & navy blue shirts (which is what all the employees wear).
Hold a synchronized swimming competition in a public fountain in the middle of New York City.
Check their backpacks into the coat check in a bookstore with their cell phones inside them. Have all the phones with the same ring go off, one after the other. Then one does a solo. Then they all go off at once in a grand finale and then total silence.
Go into a Starbucks and perform a certain set of actions, then do the same exact set of actions again and again, and again, and again.... A time loop.
I guess it's kind of like Candid Camera. Except they don't fess up afterwards and tell people what's happening, they just dissappear.