When I was a youngin, we took to the woods outside of town with generators, kegs and a couple of live bands. It certainly wasn't as hip as this party but when you get sick of getting busted in the city, you get creative. We hung lights in the trees and constructed stages for the bands.
I had this exact same thought the other day, walking past groups of people sitting on the stoop, playing music out of their cars. It looked exactly like a field party, except it was in Manhattan.
Now I am 40... You kids need to get off my lawn!