I've noticed the beta badge on Stack Overflow.
Is the participation open? How is it possible to be part of the private beta?
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The private beta was before September 15th, 2008.
Physicists are still working on whether it's possible to become part of it.
Just find yourself one of these:
Here's one way you might be able to at least get the beta badge and make people think you were part of the private beta:
You can't. The beta period is long over.