Grit doesn't have the same function as oystershell, grit is what hens use to grind up hard particles of food such as grain in their gizzards because they don't have any teeth to chew with, to break it down. You can buy bags of what's called Mixed Grit quite cheaply and this usually consists of a mixture of small insoluble bits of flint grit, for the grinding, and bits of broken oystershell, which is soluble when the chickens eat it (although it doesn't look like it) and can act as a calcium supplement. If your hens are only fed on pellets they don't really need the grit, and the calcium they need will be in their pellets, as DBE says. If your birds can free range they will probably find grit for themselves if the area has any little griity stones available. However, most people give their birds a pot of grit to pick at, because its cheap, lasts for a long time, but may be needed in small amounts to aid their digestion. They can then pick at it if they feel the need.