There is often confusion when the word "grit" is used, as there are two types:
Hard grit (insoluble grit) is the small stones the chickens swallow. They use it to grind up their food in their gizzard. If they have access to outside areas with soil, they should find all the small stones they need in the soil.
Shell grit (soluble grit) is usually broken oyster shell (although any sea shells or egg shells will do). The chickens grind this up and extract the calcium carbonate, which they use to build new egg-shells.
In both cases (hard grit and shell grit) a balanced feed will have everything they need. But if you choose to provide extra grit (of either type) you can just put it in a small container in their run and they can choose to eat as much as they need.
I am having an interesting experience at the moment where I am trying a new brand of feed. According to the manufactuirer, it is perfectly balanced and contains everything needed by a laying hen. But my chickens are currently eating huge quantities of the crushed sea shells that I have left lying around for them. They have never done this when eating any of the other brands of feed I have used. So I don't believe the feed I'm using now has enough calcium and I won't be buying it again.
In this case I'm very glad I provided supplementary shell grit and let the chickens choose their own dosage. It won't do any harm to have it available, and in cases like mine it proves very important.