Chickens who get just water and chicken food often do fine without grit.
Chickens who eat other things usually need grit to grind it up.
The grit can stay in their gizzard for quite a while, so if they picked some up from the ground in the past they may be fine.
I do not know whether your chickens badly need grit right now or not. 

 Someone suggested it, and I got busy thinking about where to get it and what might work for the purpose.
If you don't have easy access to grit right now, your chickens might be fine, but I just don't know.
You could consider getting some next time you get chicken food, if they come from the same store--it won't help if they need it now, but at least you'd have it for later. And buying it at the feed store is probably cheaper than buying it online (because grit is heavy to ship.)