In the modest black box space of The Bushwick Starr—occupied this Sunday morning by some twenty-odd performers and a three-piece band—a slide whistle signals the beginning of rehearsal.

The 74-person-maximum-capacity sign posted on the wall of Union Square Pool has been violated—probably several times over. The audience, populated in part by some of New York’s less-inhibited indie musicians, forms a spontaneous ring of folk-dancing; onlookers clap, kick up their heels, and partner with people they’ve never met before.