You may want to offer the oyster shells and grit separately and as free choice.
I also save my egg shell that I use, rinse them out, then once a week when I'm using the oven I put them on a cookie sheet and pop them in the oven for about 7-10min to crisp them/dry them out.
Crush well and give back to the hens. Mine will gobble them up.
I save all mine also, and use my dehydrator to bake them per say. Then I use my mortal and pestle to grind them up. Just make sure they don't resemble the egg shell any longer or you may get what you don't want....egg eaters. Mine gobble them up also. I put mine in a small feeder and they have it free choice. Good luck