I just offer grit to my chickens in a seperate dish and they eat what they need when they need it. Chickens only have so much room in them to get their nutrition so it is important that they get enough of the proper nutrition, adding grit may not leave enough room for the nutrient rich feed. There are proper proportions to mix grit with feed but I don't know it. Doing a search would quickly give you that. If you have black oil sunflower seed on hand, that will given them protein and also is good for healthy feathers. Anyway, oyster shell should be good for calcium, I also offer that in a seperate dish. They will eat what they need if offered to them freely. Grit and oyster shell serve two different purposes and cannot do both jobs, they need both. Canned cat food is also a good choice but not on a daily basis (at least I wouldn't think), if noone mentioned this so far. Crickets, meal worms from a pet store are good as treats, but can be pricey if buying alot. Buying a dozen isn't too expensive though.
HTH
Carolyn