At least one group in Washington has escaped the long arm of the Lewinsky scandal: AmeriCorps. The national service program is a favorite of President Clinton’s, and its supporters were worried that it might be a target for his congressional critics. But the four-year-old program has won over skeptics by emphasizing its grassroots structure and its alliance with respected nonprofit organizations around the country. Members are selected by and work for such groups as Boys and Girls Clubs of America and Habitat for Humanity. AmeriCorps prevailed on a key Senate funding vote in July by 58 to 37, winning over such former Republican critics as Rick Santorum of Pennsylvania and Daniel Coats of Indiana. On Friday, Clinton will preside at a White House swearing-in ceremony marking the program’s 100,000th member -— a benchmark the Peace Corps took more than 20 years to reach.