@kwendy I feel this shouldn't be too surprising. It seems she was E-ranked in her little town because she was a mixture of new and not too adventurous, so she didn't take dangerous quests in order to increase her rank quickly. Because it is a small town, it's probably the kind of place where everyone has met, thus, they know each other on some level. This makes it pretty hard for people to be mean to each other and not be known. After that, the 'hero' used the charm to get her in the party, leaving her VERY unaware for some time because charm is probably really bullshit.
@esp It is possible that they only get excommunicated from THAT guild. While the guild itself is shown to be trustworthy a a result, this still wouldn't fix the extremely scummy adventurers.
@Akabane If I'm predicting this correctly... the 'hero' is just a self-proclaimed one and he used charm in order to make people believe this was the case.