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.
If you don't have anything of such things left in your hand, it's not you a member of its community anymore, and there is nothing left for everyone to do in case of something happening. All you can do is to start from the beginning as a new applicant.