Unfortunately this happens all too often. I used to frequent DPreview(I left because there wasn't enough photography and too much equipment talk)and there were several instances where people had photographs stolen and blatantly posted on other sites as someone else's work. The biggest difference with you Alison is that you know your thief. I understand your wanting to disappear Alison, having something stolen is very personal and will leave you feeling a little violated. I had my car broken into a few years ago, and the whole thing seemed surreal, I felt worse about the act than I did about the stolen property.
If you look at it from another angle, it's flattering, your work is good enough that someone stole it. Hopefully you'll get this resolved amicably, the person who gave permission to use your work probably thought they were doing you a favor. The strange thing is, the wedding photographer should have known better than to use one of your shots without your permission.
Sounds like your portraiture group needs a session of ethics and ownership, rather than picking on you because you want to protect your property. That's very small of them to boot you out, you are right here, not them. A photograph is your property, and for someone to use it without your permission is just flat wrong, and against the law. Hopefully it won't come to involving the law, and certainly that's up to you if you want to take it that far.
We love you here Alison, keep posting when you feel up to it.