Make sure you build in time to refrigerate the gin, cranberry, and sugar mixture at least overnight.
Combine cranberries and sugar in a small saucepan over medium heat. Cook, stirring occasionally (do not break berries), 7 minutes or until sugar dissolves. Transfer mixture to a large bowl, and let cool completely. Add gin; cover and refrigerate 24 hours.
Pour cranberry mixture through a fine wire-mesh sieve into a bowl. Reserve berries.
Combine 4 ounces strained gin, 2 ounces (2 tablespoons) Campari, 2 ounces (2 tablespoons) vermouth, and 4 dashes bitters in an ice-filled cocktail shaker. Shake vigorously for 10 seconds, and strain into 2 cocktail glasses. Garnish with orange rind and a few reserved berries. Repeat with remaining ingredients.