I'm not saying it's easy to rebuild a rapidly crumbling community, I'm saying the two things they are asking for are unrealistic and I hope they find a better way. I have neither of those things. I don't know why I would keep an invite email, or have a screenshot (unless I was a part of a invite trading community.....)
The idea though is to potentially take a screenshot of the client. It would show all the tracker info, along with the user ID embedded in the announce. Thus proving membership.