I'm a fellow newbie, so hopefully some more people will chime in... my understanding is that your chickens won't need grit, they'll eat some of the sand and it will grind their food for them in the stomach. Oyster shell is just for added calcium, to make the egg shells stronger. So yes, offer oyster shell, but you probably won't need to offer grit. Does this sound right?
I would not think they need grit if you have sand in their run. Mine free range and have never needed it. When it comes to oystershells some people will say yes, and some say no. Again, I don't give mine any. I just buy them a qaulity layer feed and I have never had soft eggshells. But, it doesn't hurt! You could leave a small dish of it in the coop and if they want it they'll eat it!
I know many people feed primarily scraps and the birds forage a lot, that's the situation that would need oyster shells. THe formulated layer pellets/crumbles are formulated with the "right" amount of calcium.NO extra needed!