More than 100 stars in the entertainment world have formed Artists United to Win Without War and plan to send an antiwar letter to President George Bush today. The signees include Martin Sheen, Barbra Streisand, Kim Bassinger, Harry Belafonte, Matt Damon, Uma Thurman and Danny Glover. Edward Peck, the former United States ambassador to Iraq, is among some of the non-Hollywood signatories.