It depends. No. Not yet.
They don't need grit if the only thing they eat is poultry feed, whether crumbles or pellets. If they free-range, get vegetable parings or table scraps, or get scratch, then they may need grit depending on what the ground conditions are like where you are. I chose to hang a 3-pound feeder half full of grit in my chicken house. It may not have been strictly necessary where I am, but I don't have to worry about it now.
Oyster shell will disintegrate and dissolve in the gizzard, especially if you are giving your chickens grit. Grit won't. As soon as my semi-local feed store gets more of those 3-pound feeders in, I'll be hanging another one that's about half full of oyster shell. I anticipate the oyster shell will wind up replenished three or four times for every once the grit is refilled.
The chicken uses calcium to create the eggshell. Layer feed contains enough calcium to replace the calcium the average chicken uses to make an egg. Until the chickens start laying, they don't really need the extra calcium. I would wait until your birds started laying before I put oyster shell in the coop.
RSD