You can provide grit if you wish, it won't hurt anything. But they should be getting all the grit they need from the ground. You almost certainly do not need to provide grit.
You should not give them any extra calcium until they need it for their egg shells. That is when they start to lay. There are plenty of studies that show that feeding excess calcium to chicks can damage their internal organs. In these studies they start off feeding them layer rations which has the extra calcium in it and they cut the chicks open to look at them internally to see what damage has been done.
You could probably offer them oyster shell and it should not cause any problems. That's what many of us do when we have a mixed age flock, feed them all Grower, Developer, Flock Raiser, something like that that does not have the extra calcium in it and offer oyster shell on the side for the hens that need it for the eggs. The ones that don't need it won't eat enough to cause themselves any problems. But I think it is just good practice to not offer them any until you have some that need it.
Grit has nothing to do with oyster shells.