To fill vacancies, Blackstone Group LP, the biggest U.S. office owner, is going after startups and their armies of twentysomething workers. In Flaherty’s building, 100 Summer Street, Blackstone has installed a private club on the second floor open to employees of companies in the building, with beer and wine taps, a gym with private showers, arcade games, foosball, and baristas who finish off cappuccinos with foam artwork. It even has a startup-style name, Assemblyon2. Les mer her