My chickens are picky. They won't eat them unless I discuise them as a treat. I cook rice and I put the egg shells in the ultimate chopper and turn them almost to powder. It's more like sand. Then I mix the 2. As long as I don't add too many egg shells they will eat it right up.
I dry mine for a day or so, then bake them in the oven. I crush them with a glass or something, whatever is handy, and then mix it with their feed (I use either crumbles or pellets), with their hard boiled eggs I crush up for them, their spaghetti, or their brown rice. I've even just set a bowl of crushed shells for them to pick at. They love them and will eat them before they will eat the oyster shells.
I just save them in a coffee can when it full I put them in a gallon bag crush them with a rolling pin then put them in a pan and put them in the pen. The chickens then eat what they need. No problems with egg eating and no extra cost of oyster shell.
Mine go straight back out to them in the compost pile along with the rest of the kitchen scraps. No crushing, no baking, no nothing. They tear them up with no trouble. I mean have you seen those beaks?
Those girls would tear me apart if they were big enough!
This doesn't seem to cause them to develop any kind of egg-eating habit, have been doing it for 2+ years now.