Be careful what you ask for, you enter the realm of Bruce Almighty. I'm sure he has access, but I don't think we do. Of course, you could just ask the member! I believe this site was created on March 20, 2008, so all of us are newbies here. I joined May 23, 2009. I am about 15 months old.
A really good way for non administrators to keep up with this would be to make it a point to welcome every new member. Then just go back through your comments till you find when you welcomed them. The going back through would be a lot of trouble but would work. The welcoming would be easy, and might get you a lurker who turns out to be a lunker!
Every member has a page and any member can go to that page by clicking on the avatar picture. On that page is a "comment wall" and you can scroll down until you find the first message. That will usually be a welcome from Bluegillboogieman. Sharon, your's was Nov 4, 2009 at 6:07am, so you probably joined on Nov. 3, kinda late in the evening.
BTW, be careful what you say when you comment directly to someone, since everybody else can see it too. ;-)