PHILADELPHIA: "The era of big government may be over but the era of big challenges for our country is not," President Clinton said Monday in front of Independence Hall in Philadelphia. "So we need an era of big citizenship." At the Presidents' Summit for America's Future, big citizenship meant big names. Former Presidents Bush and Ford and former first lady Nancy Reagan, representing President Reagan, lined up on stage to sign a declaration of a "call to service" on the same site where delegates once signed the Declaration of Independence. Jimmy Carter spoke by cell phone. Yet on a day on which politicians gave freely of themselves, President Clinton's showcase volunteer program AmeriCorps joined his Administration's scandal club. Even as Clinton announced a plan to increase the number of scholarships available for AmeriCorps workers over the next five years by asking corporations to pitch in, the organization is scrambling to answer why five highly paid political appointees stayed on the payroll after their jobs were abolished--and why one of them wrote John Huang to arrange a meeting between a Clinton official and Asian-American business owners in Los Angeles.