Hmm I can only think that everyone's definition of "friend" is different? Maybe that person was just trying to help you avoid disappointment, in that, as you know, people with mental health issues are often wrapped up in self and not really careful of hurting other people's feelings and the like when they are?
As for the messages, I would ask the admin what's going on. It surely does sound like a clique-ish board if it isn't a technical glitch.

But, so many support groups IRL are like that...unfortunately. This behavior is probably the reason others make their own group, only to find that they are exhibiting the same behavior and enjoying the "core group" only?

Try not to take it personally. (I know really hard to do.) If they are mistreating you, it's their problem, and not yours.

I hope you find the support you need, someplace!