It’s the pink mommy club of the Upper East Side. But only the well-connected seem to know it’s there.  The Pearl — part fitness studio, part social club, part multigenerational event space for personal development programming and bachelorette parties alike — sits in a building like any other on a street like any other on the Upper East Side.  There’s no signage or reason to believe taking the elevator to the fifth floor will bring you anywhere other than a dentist’s office or a tax consultant’s cubicle.  Inside the 5,500-square-foot space, however, the ceilings are high and the feminine energy…