My grandma used to collect the egg shells in a old coffee can on the cooler end of the wood cooking stove(to let them dry). She ground them up with some old wood tool and threw them in with the feed.
We just throw them into the run as we get them, no preparation necessary, and none of our hens are egg eaters. The girls can go find their eggshell pile whenever they want eggshell instead of oystershell calcium.
I actually cook our eggshells to feed back. I know a lot of people don't bother, but have you ever smushed up baked eggshell with a wooden spoon? It's very therapeutic. I cook them in a metal bowl, just throw them in while the oven is cooling off after cooking, and then set the bowl on the counter with a wide wooden spoon in it. Everybody takes turns crunching them up as they sit at the bar while I am cooking or they are eating breakfast or whatever...just crunch eggshells and chat, lol. Fun times.