Some people, like Neville, had paid up just to stop Hermione from glowering at them. A few seemed mildly interested in what she had to say, but were reluctant to take a more active role in campaigning. Many regarded the whole thing as a joke.
The clause might be parsed either "[what she had] [to say]" or "what she [had to] say" in my opinion. If we take "had to" together for comprehending, I think it might have a different meaning other than 'must'. I found this dictionary definition might fit for this context:
have to: used to suggest that an annoying event happens in order to annoy you, or that sb does sth in order to annoy you.
Is it what it's supposed to mean in this context? How should we understand it?