Grit is what "grinds" the food in the gizzard - so for anything soft (yougurt, milk, cooked oatmeal....) it isn't necessary. If you are feeding cracked grains (oats, corn, etc) it is needed. When you see birds at the side of the road, they are often picking up gravel or"grit". Chickens and other birds will pick it up in the dirt. Some people who free range don't bother with grit.
Most people offer it free will.
There is such a thing as "chick grit" which is just smaller than regular grit. You can offer sand, as suggested, and pick up some grit later.
For chicks, I sprinkle it like salt on their food til they grow up and choose it on their own (I keep a container of it next to a container of oyster shell for calcium - and the chickens take it when they want it. They sure use more in the winter when they can't get to bare ground.