OK Go may be a four-piece group, but in their performance Friday night at Mercy Lounge, a sold-out crowd of 500 people might as well have been added to their lineup. The band, touring in support of its new album “Hungry Ghosts,” took the stage for almost two hours and never once allowed the energy to decrease or the crowd to not be entertained–or, for that matter, not be involved. “It was great,” said lead singer and guitarist Damian Kulash. “We had a fun time–sweaty and deli