THIS IS IMPORTANT!!
grits are a food. if they are cooked, you may give them to chickens.
***but what you are probably referring to is Chicken Grit. this is crushed granite bits. chicken do not have teeth and to break up certain food, they must have this grit. chickens kept in coops most of the time will need this mixed in with their food and in a separate container for free choice. chickens that are allowed to free range, usually pick up small bits of rocks that do the job as well. however, i would offer it to them free choice in a separate dish too. if chickens do not get grit, they will get what is known as "impacted crop" or "sour crop"... this means their food is not broken down and gets stuck in their crop. it can't be digested and they WILL DIE.
oyster shell should be given free choice to laying chickens. THIS IS NOT A SUBSTITUTE FOR GRIT! this oyster shell provides calcium that laying hens need to lay eggs with hard shells. the oyster shell is SOLUBLE and will not do the job of breaking down food.