Barn lime is not grit. It can be used sprinkled sparingly in the coop to help with odors.
Grit is Crushed Granite - it's an insoluble form of grit that birds use in the gizzard to grind up/process foods.
Could the store have been talking about Oyster Shell? That can be given free choice to help supplement calcium when birds are in lay.
Both Oyster shell and Granite Grit should be offered free choice - meaning place it in the run and let the birds pick up what they need.
Where are you located in the world? Sadly, most feed stores - especially chains, the employees do not know what certain products are nor do they know the purpose of the product.