Agnes, that automatically happens when you send a story to a publication and the publication makes a change or leaves a private note. I just hid it, because I actually didn’t collaborate on this story. Sylvia is the sole author. Sylvia sent it to my publication One Minute Wit and we decided it wasn’t quite right for my humor publication. Unfortunataley, the collaboration “thanks” remains, even though I didn’t actually collaborate or publish it. All you can do then is hide the “collaboration”, so I just did. It’s one of those weird Medium things I wish they would fix.
There is no way to add a collaborator (or proofreader or editor) to a story, that I know of, other than typing it in yourself somewhere.