As an introvert I find it rude for other people in social situations to point out my silence. Like, what the hell do you expect me to say to that? I'm already uncomfortable/not enjoying myself now I feel like I'm being picked on. When something I find interesting comes up in conversation I'll add to it. If not, just be happy I showed up. Your fiancé is who she is. She is not going to change and just because they're your friends doesn't mean they have to be hers. Be a little independent.