Go to a private beta site, such as Spanish.SO. You need an account on SE to get in.
Now, click on the StackExchange logo in the top left corner and press either "Stack Overflow en Español" or "Stack Overflow en Español Meta" or click on the Stack Overflow en Español logo above the heading " Confirmar la nueva cuenta".
It directs you to the site homepage with questions, giving an outsider free access to the private beta.
I know about this question, but this is letting someone even without a SE account go to the site.