So if I understand this correctly, you had a problem with a supervisor, as did a few other people, and now someone else entirely wants to write a letter of complaint on behalf of all of you?

I agree with the poster above. This is them, not you. I've run into situations like that before and there seems to be a sort of 'type' that does that kind of thing. I would try to figure out how you can identify those types and maybe keep them a bit distant.

It could also be that you are giving off an air of someone who needs to be rescued because you are scared/shy/fill in the blank, but I really think this has more to do with the people you are surrounding yourself with. I hope they just drop the letter.