I'm with the group that doesn't want to force them to eat something they don't need. I offer oyster shell on the side so those that need it can get it and those that don't need it aren't forced to eat it. I know they will try to pick around it but they are not always real successful doing that with the finer stuff. I find that the ones that don't need it might experiment with it a bit but really won't eat much.
There are lots of different ways to offer it. I took a large plastic yogurt cup, cut a small hole near the bottom so it would drain, and wired that to the run fencing about back height on a hen.