English lets one name cover a whole people when you put the in front of it: the Pawnee, the Navajo, the Cherokee. No -s is needed, and the verb that follows is plural — the Pawnee are, not the Pawnee is. That plural verb is doing real work: it says these are people, many of them, living now. The formal name used today is the Pawnee Nation. Write it with a capital P.

Here the word points at one person rather than the whole people, so it takes a and it has a plural. The difference from the other use is only the article and the number: the Pawnee are farmers is about everyone, a Pawnee farmer is about one. When speakers introduce a person today they very often use the word like an adjective — she is Pawnee — or add a noun — a Pawnee writer. Saying a Pawnee on its own is fine, but it can sound as if you are naming a category rather than a person.