There is a silver badge Beta, it says Actively participated in the private beta.
What is the private beta?
Ask Ubuntu once started off as a simple proposal on Area 51. During which people helped to shape and define the parameters which the site would operate under. Once this was defined the proposal went through a commitment phase to see how much traction it would gain and test how many people were interested in this idea. After that Stack Exchange launched https://askubuntu.com/ - a private beta to see if the site could survive a public beta launch. As you can imagine it then launched into public beta where anyone could join and shortly after public beta became the site you see today; full launched as Ask Ubuntu.
You can check out each part of the phase, from definition to launch on the Area 51 profile.