I would guess that somewhere in the neighborhood of 100,000 people have been qualified to wear this t-shirt. Approximately 99,950 of them paid for the privilege of membership, and the other 50 would have gotten the joke. Of those, perhaps a dozen will still remember it.
[and no, it’s not actually funny unless you’re one of those dozen, so I’m not going to explain]
[Update: Rory suggested a slight modification to improve the reference, and further decrease the number of people who’ll get it]