Combine the grapefruit juice, elderflower liqueur, Campari, and simple syrup to taste in a pitcher.
Fill 4 wine glasses with ice cubes, add some of the grapefruit mixture, and top off each glass with sparkling wine. Serve immediately.
To make simple syrup, bring equal parts sugar and water to a boil in a small saucepan, stirring until the sugar dissolves. Cool and chill.