Keep a good balanced ration freely available to your birds then let them forage for whatever they can find. For non-meat bird hybrids on good range they can often find a pretty good bit. It won't be all grass, but a combination of grass, other kinds of tender plants, and whatever insects and small life they can find. This typically can supply a fair amount of protein, vitamins, minerals, and some fats, but is usually short on carbohydrates. When range is good their consumption of complete ration may go down though they'll often eat a fair amount of grain if you provide it. When range is poor they'll eat more of the complete ration. Meat bird hybrids have such voracious appetites they will gobble up everything.
Chickens can only eat and digest just so much grass, but if they are free ranging they are never eating just grass anyway. Be sure to keep a good ration freely available to them though so they can still get what they need should they not be able to forage it for themselves.