By the time you see symptoms, all the other birds have already likely been exposed. But your bird might just have the flu and be fine. You won't know if egg production has been affected until you start seeing eggs. Sounds viral to me, so antibiotics wouldn't help in that case (but I'm not an expert!). If you do want to treat with antibiotics, I'd really consider getting a vet to look at her first. And next time, I'd recommend separating a sick chicken at the first sign of symptoms, it may not prevent the others from getting sick, but you can provide her with better care. Such as, extra heat if needed, peace and quiet from the other chickens, special diet or medicine, etc.
And, no, I don't think you'd need to get them extra grit. As long as they are pecking at the ground outside, they should be getting grit on their own.