I'm pretty sure everyone can join; how does one have documentation proving their Muslimness?
It's a convert thing, when one wants to marry a muslim/ah in my wife's country the outsider must provide proof of their islamicness or bear witness at the local islam instatution. So I went into a strange building and said strange words infront of strange men but don't quote me on that, people might get the wrong idea.
And as far as I know, appostacy is not an option in that place but rehabailitation/jail can be. I'll not mention the country by name at this time in an attempt to preserve my annonimity, nor will I confirm or deny any guesses.