When the Summerfest headliners were originally announced, Pentatonix was one of the most intriguing names headed to the Big Gig. I wanted to know how a five-person a cappella group, one that relies on insanely tight harmonies and arrangements, would fare in an outdoor festival environment. As it turns out, I and the ecstatic young crowd gathered at the Briggs & Stratton Big Backyard Thursday night had nothing to worry about.