President Barack Obama is welcoming former President Bill Clinton to the White House to mark the 20^th anniversary of the AmeriCorps national service program.  They will address hundreds of volunteers being sworn in for duty at a ceremony today on the South Lawn.  Clinton signed the bill that created AmeriCorps and launched the first class of volunteers on September 12, 1994, exactly 20 years ago.  The federal agency that runs AmeriCorps says that, since then, more than 900,000 people have spent more than 1.2 billion hours volunteering on projects nationwide.  Several students from Nashville and the surrounding areas are in AmeriCorps through Kaskaskia and Rend Lake Colleges.  Swearing-in ceremonies are also being held in the 50 states and several U.S. territories.  The events kick off a year of service for some 75,000 AmeriCorps members.