Chill 6 champagne flutes. In a small saucepan, stir the sugar and 1/2 cup water until the sugar dissolves, about 5 minutes. Add the cranberries and bring to a boil. Lower the heat and simmer for 2 minutes. Strain, then refrigerate the syrup and cranberries separately.
Place a few cranberries in each flute and top with 1 tablespoon cranberry syrup; fill each glass with prosecco.