Infinity is a disco in Lower Manhattan. It is 3 a.m. on Friday, the dead of night, but the evening is not yet half over and there are still two thousand people here. Perhaps a third of them are crowded against the shoulder-high banquette that rims the floor, from which they can watch those parading before them or simply stare at themselves in the wall-to-wall mirrors that reflect unending images across the huge loft space. The mirrors are part of the show at Infinity, for not only can you watch your own image change beneath…