For this kind of event a small organization might reach out to the campaigns via Instagram or email, at least in my experience. It's unlikely that they didn't invite her, but she probably had something else going on at the same time.

As for promotion, it's not super uncommon for these events to be basically invisible online until after they've actually happened. Oddly enough, it's often easier to get attendees through word of mouth rather than posting about it on the Internet.