I just leave a rubber bowl in the coop filled with oyster shell and let them eat what they want. I don't feed grit because mine free range but you don't need to feed grit if you are just feeding layer food but if you give them other treats they need it to grind the food. Just throw a few handfuls around and they will take what they need.
With layer feed, your chickens should need neither oyster shell nor grit, Rosaline.
Layer feed is made with sufficient calcium. The extra calcium in oyster shell will not be needed unless they are eating other foods.
Since the feed is milled, your chickens do not need grit. Once again, if they are eating some other foods, they may require grit or, at least, access to sand.
Have fun with your new birds