Using a sharp knife, cut all the peel and white pith from the grapefruit, holding it over a bowl to catch the juice. Cut down the side of each segment in turn, as close to the membrane as you can, and ease it out. Squeeze any remaining juice from the membrane, then discard it.
Cut the grapes in half if they are large, otherwise leave them whole. Add them to the grapefruit segments and juice. Chill for at least 30 minutes, or overnight if that is easier.
Spoon the sparkling grape juice over the grapefruit just before serving. Serve the fruit on its own, or top with yoghurt.
Variations: if you cannot find sparkling grape juice, use still grape juice and add it to the fruit before chilling. Orange segments can also be added, or used as a substitute for the grapefruit.