“Barbershop’s A Poppin',” at the Swing Auditorium in Saa Bernardino, May 21, will be oneof the largest performances of! four-part harmony. Groups will include the Osmond Brothers ofthe Andy Williams TV Show, international champion “Sidewinders,’’ the 1962 champion “Gala Lads,” the “Dapper Dans” from Disneyland, and the international ;hampibn “Citrus Belters” of 65 voices.