Grit is used to help the gizzard grind up the birds' food. If you are feeding a crumble or pellet I don’t believe grit is necessary. If, however, you are feeding whole grains, mash or scratch feed you may want to offer grit. Free range chickens should find all the grit they need.
Oyster shells are used to supplement calcium in the birds diet.
If you feed a name brand layer feed there should be no need for oyster shells. Thin egg shells can be a sign of low calcium but remember the older the hen the thinner the shell.
We use neither grit or oyster shells. If it makes you feel better go ahead and use grit and oyster shells. It won’t hurt your birds.
As always, post further questions in the comment section.
-Dale, aka Turkeyman