Archivists are the specialists who snatch objects from oblivion. They have long spent their careers cloistered, like the objects they protected. But now many of these professionals are stepping out. A main reason is the Archivists Round Table of Metropolitan New York. The group, which recently surpassed 500 members, holds monthly events that draw a young, well-dressed crowd, hungry for chances to network, train and socialize. Members not only work at libraries, where archives have long resided, but also at such organizations as the Fashion Institute of Technology, the Junior League, the Episcopal Church, the Philharmonic, the Stock Exchange and the Brooklyn Navy Yard.