The 'Beta' badge is gained if a user actively participates in a private beta.
But I don't know how and where to participate in a private beta.
What is a private beta site?
I have 3 stack exchange beta accounts(Islam, Sports, Askdifferent).
I understand that they are public betas rather than private ones.
I found this question but the explanation there was not helpful.
What is the difference between a private and a public beta?
How do I get an account in a private beta?