Your example is actually a special case. "Untouchables" without an article refers to social or cultural outcasts in AmE. But "the Untouchables" (with the article, and whether capitalized or not) were a group of special agents of the U.S. Department of Justice led by Eliot Ness during the Prohibition era. They were famous for capturing many notorious gangsters, including the legendary Al Capone.
Otherwise, AmE generally uses articles for defined groups (the homeless, the disenfranchised, the upper middle class, etc) just like our BrE counterparts.
