In the bottom of a large fruit bowl, crush half of the mint leaves with the sugar and add the orange juice and Grand Marnier using a wooden mallet or spoon to muddle it all together. Add the berries and toss well with the other ingredients. With a potato masher, mash the berries slightly, and toss again. Strew the remaining mint leaves over the top of the berries, cover and let marinate refrigerted for at least four hours. Decorate with extra mint.