Ground oyster shells or egg shells are a great source of calcium. My chicken feed has calcium built in, so I do not have to suppliment it. If the chickens do not get the calcium they need, their bodies will start consuming their own bones for the egg shells. You will get chickens with weak bones and thin egg shells.
I usually just put them on a cookie sheet in the oven for about 20 minutes at around 200 degrees. Kills whatever germs, but really, it's just easier to crush them up small when they are really dry like that.
We let them dry in an egg carton, grind them up in the blender and keep in a lidded container. I add add a handfull to their scraps. You'll know if you're feeding too much because their eggshells will get alot thicker and be difficult to crack. When that happens, I back off on the eggshells. Has worked well for us so far.