I am another who thinks egg shells don't provide quite as much calcium as oyster shell. I offer it in a separate container and they eat layer feed, just nibbling on the oyster shell when they need to. But I do like to boil up any older eggs I have in the fridge, crush 'em up shells and all, and give those to the girls a couple of times a week. Granted, with a lot more chickens I probably have more older eggs - eggs that I don't want to sell because they are more than 2 weeks old.
For me the bonus of doing this is that they get the egg shells, but they also get the protein from the eggs and I like that this time of year since many of mine are currently molting. I have never had a single "egg eating" issue doing this. What I toss in their pen has no resemblance to what they are leaving behind in the nest, so it hasn't caused any bad habits. I usually boil them up and when they're cool I put them in a plastic grocery bag, twist it shut and pop it back in the fridge. When I'm ready to feed them, I tuck that bag into a second grocery bag so I have a double layer, lay them on the countertop, and start "smushing" with my hands. Once the shells have been cracked, it's like kneading bread. Doesn't take long and they are all crushed up and ready for the girls. They love 'em.