This is a horrible situation for you, OP.
The longer you leave it, the more you lose influence and Wendy gains power. (And I have no problem calling her Wendy, btw, given what a woman who is actually called Wendy has done to our family in rl!)
At the moment the group know you better than they do her. But people are fickle, and busy, and don't always see beyond their immediate concerns. So if you allow yourself to be sidelined, and not get involved with the others so much, they may assume that that is what you prefer/just how things go/etc.
Re the letter. You will need to play it very carefully if you show your friend. I know you haven't shown her yet, so as not to upset her, but if you showed her and emphasised how there is not truth in it, how much you like her, etc, and don't try to infer who you think sent it, you should be safe. Be open, honest and kind, and leave it to the others to start to make connections.
The problem if you don't show the letter is that, especially as it's printed, it would be so easy for Wendy (if she is the sender) to send herself one, just like yours, talking about anyone in your friendship group. She could do just what people on here are suggesting - mention it to other friends, and 'wonder' who sent it. Then it's too late. If you come out with your own letter after that, you can easily be made to look like the suspicious one.
I would show it to your friend and emphasise how much you care about her and value her friendship, and perhaps together you can decide to do what Inertia suggested and then contact the others to see if there are any other letters out there. That way you make contact with your own friend, and have her as part of your team (which I suspect would be how Wendy would see it - what is wrong with these people?) before widening it out to the group.
Sorry for long post. Good luck.