Improv Everywhere are the group that did the "Frozen Grand Central" video that became such a hit a while ago. They've also done synchronized swimming in a public fountain, a musical show based in a mall food court and several others.
This appears to be their latest prank. They visited a little-league baseball game and made the kids feel like it was a major league game, complete with an NBC Sports jumbotron, screaming fans, hand painted signs, and even the Goodyear blimp.
Bravo to these guys: