Oystershell is oystershell - not eggshells. You should be providing oystershell to your chickens for calcium to build up their shells. You may also save any and all eggshells (once they get stronger), dry them on the counter for about a week -10 days and then crumble them in a freezer bag and then mix a tablespoon into their feed a few times a week in addition to the oystershell. Some people bake their eggshells, but its not necessary, its more a preference thing.
Oystershell is given freely - just like grit. Are you providing free range grit as well since you are giving them table scraps and grains? They must have grit to process grains or they will become sick.