Picking out a public beta from http://stackexchange.com/sites, I reach the site's main page and click "Join this community" in the upper right. The page refreshes, but still shows "Join this community" and I haven't apparently joined.
The workaround is to click "Ask a Question," then in the lower left click "Log In," at which point I see:
Confirm Your New Account
You are about to create a new account on Genealogy & Family History Stack Exchange using a login from
We will automatically link this account with your accounts on other Stack Exchange sites.
Then there's a big orange button which reads "Confirm and Create this Account" and clicking on that works as expected.
However, "join this community" should also work.
I'm logged in with SE and SE only (i.e. not authenticating with Facebook or Google).