Cut peel from grapefruits into ⅜” strips leaving ¼” white pith attached. Place in a small saucepan; add cold water to cover. Bring to a boil, then drain; repeat twice.
Bring peel, 1 cup sugar, and ½ cup water to a boil in same saucepan; reduce heat and simmer until peel is translucent, 15–20 minutes. Drain; transfer peel to a wire rack and let dry, 2–4 hours. Toss in more sugar.