I give mine our sandy soil from outside. I give it starting the day before I plan to give them treats. You can give grit as soon as day 1 in the brooder, but if your only giving comercial feed then they dont need it until they are fed something else. If you dont have a good amount of sand in your soil some you can buy grit at places like tractor supply. I personally prefer ground dirt since that is what they will be moving to from the brooder and it will get them use to whatever is in the soil.
In 5 or 7 years.....I have NEVER provided any "GRIT". My chickens spend their days outside in a fenced area with BARE GROUND. They can get their "grit" from the soil. ( Although I HAVE dumped "excess sand" left over from building projects into their fenced area.)
IF your ground HAS rocks/pebbles, etc. -----I don't think that you need to bother with "grit". They'll find their own...it seems.