http://www.post-gazette.com/pg/06183/702891-85.stm This sums it up pretty well, I think.

From the article;

"Organizers were hoping for 200 or so participants in the afternoon sing-along, but it appeared they had attracted two to three times that number to a grassy spot near the park's main stage. At a few minutes past 5, the cameras were rolling for the first of six takes of "Won't You Be My Neighbor."