What's the largest group of people to which you have ever belonged? Caveats: the human race, your ethnicity, your religion (or lack thereof) and the population of your country do not count.

Randy, I number myself among those individuals who have never set eyes on a live Thylacine. Yes. I only know of one living person who has, but there are probably still a few more out there. But whether this answer qualifies as the ultimate satisfaction of the parameters you have set, without resorting to outright silliness, remains an open question.     

The US Navy.

And you of course, the Marines.

I have been a pupil in a school. Thankfully a lot of kids get to be in that group by now, even if not all of them like it ;-))

I was in the Air Force once. They tried to put me in jail when they realized I was not going to re-enlist. I told the commander "What's bothering those people is that I'm young and happy and getting out, and they are old and miserable and think they can't support themselves if they get out."

At its peak, I was one of 12 million active players in World of Warcraft.

I worked for a behemoth and imagine that is the right answer. 

According to the US census, I am one of approximately 139 million people who possess all the qualifications to be president of the United States.