Add grapefruit, orange, lemon and lime juices to a large mixing bowl. (If using fresh juices, strain out any pulp first.)
Stir in the simple syrup. Taste. If too tart, add more simple syrup 1/4 cup at a time.
Chill for at least one hour.
Before serving, transfer to a pitcher or punch bowl. Add the prosecco. Garnish with citrus slices.
When ready to serve, add 3 to 4 ice cubes to a glass. Fill glasses with punch. Top up with more prosecco if desired.
Garnish glasses with citrus slices and serve.
Notes
Make your own sour mix by combining 2 cups simple syrup (a combination of 1 cup sugar and 1 cup water, heating or agitating until the sugar dissolves) with 1 cup lime juice and 1 cup lemon juice.