I have poked around in the numbers below 40,000, and the results are odd.
There are some closely below that no. from May 2007. Further down, from around 39,050, the number show more than 8,000 users, probably all from June 2007. But, 8/07 and 9/07 are in the area, too.
WGTalex (30,557) and WGTadmin (30,558) have been added later, they joined 2008, apparently.
From there, it's not far down to the lowest number, albeit from 2010.
For the sake of their privacy, I won't unveil how I found them.
So, for now, May 2007 seems to be the earliest date.