Throughout the game, each class will receive some crew free as volunteers. Nanu Nanu is one of the earliest examples of this pattern - quite possibly the first. But all classes have these free volunteers from time to time. But after the class where a crewman joins one class as a reward, the same crewman is "unlocked" in the Crown Shop when pirates from the other classes complete that quest.
There are a few examples where everyone, regardless of class, receives the same reward after completing a certain quest. Bonnie Ann is the first example. Not too many of these, though.
And there are a few who can only be bought in the Crown Shop.