I first heard about the NPL when I was in middle school, I believe. I saw an article about it in one of the back issues of GAMES (the school library happened to have a subscription and I loved it). However, it was years before I finally joined. It took some prodding by people I met at the Mohonk Words Weekend who were already members – WILLz, IRBS, Non Sequitur, etc.
Source: npl-folk email from QED.
//First Issue:// Mar 1994
After I decided to join the NPL, I figured I had to decide on a nom before sending in my money. I went through a few choices from Greek mythology (one of my favorite non-math or puzzle subjects) but nothing really came to mind as appropriate. Finally, I thought “Maybe I should call myself QED (after the abbreviation seen at the end of a lot of mathematical proofs) since I am a math major.” I wasn't coming up with any better ideas so I decided to go with it. As an afterthought, I figured it would be great to be able to end letters with it as well.