Technically there are no member numbers. There is however a sequential number assigned when you register on the National PGR website. There is a way to see that number which I think you can do by clicking on the posted by box or screen name and it will show the number but its been forever since I've tried doing it.
After you've logged in,click on your name in the upper right corner just left of the word logout. Then up in the address window you'll see a number near the right end of the address. That's it, nothing to it.