If they have access to the ground, they will get their own grit. If you buy it, it is the leftovers of quarrying granite. They screen the residue from quarrying to get pieces the right size. It is not treated with anything and is about as "organic" as you can get. It is just pieces of rock.
Chickens do not have teeth, so they cannot grind things up the way we do. So they ingest small pieces of rock and store those in their gizzard, where they are used to gring up their food. If all they eat is prepared commedcial feed, you do not need grit. Even the pellets have already been ground enough. But if they eat anything else, they should have grit.