Eggshells are great to feed back to the hens for calcium, just crush them first, it will not teach them to eat their own eggs.
But,it isn't enough calcium for them, it would be best to also give them oyster shell on the side.
As to grit, oyster shell works a bit like grit... but not really, because it breaks down.
If they free range and have lots of dirt/sand/etc. they probably don't need grit on the side, they will find their own.
If however they don't have as much area to roam, or it freezes up (so the ground freezes hard with no more access to grit), then grit on the side is very helpful. They need the grit to properly digest their feed.