Chill a 4-quart pitcher and 8 mugs or pilsner glasses in freezer about 1 hour. Once pitcher and glasses are frosty, slowly pour 3 (12-ounce) bottles chilled pale ale and 3 (12-ounce) bottles ginger beer into pitcher. Add 4 drops Angostura bitters and stir briefly to blend. Place a lemon slice in each glass and fill with shandy. Serves 8.