Recipe from marthastewart.com.
1/3 cup sugar
3 1/3 cups grapefruit juice, preferably fresh
2 cups dry vermouth
3 cups supremed grapefruit segments
1/2 cup thinly sliced grapefruit wheels, plus more for garnish
Bring sugar and 1/3 cup grapefruit juice to a boil, stirring until sugar dissolves; let cool completely. In a pitcher, stir together syrup, remaining 3 cups grapefruit juice, vermouth, and grapefruit segments and wheels. Cover and refrigerate at least 1 hour and up to 1 day. Serve over ice; garnish with a grapefruit wheel.