Ask HN: Best way to attract private beta users?

Does anyone have any tips on how to attract people to sign up for a private beta?

I've read that it makes sense to try and get at least five hundred signups if you want to get enough feedback for a private beta to be useful.

Ideally I'd like to send invites out in a staggered fashion after each iteration during the beta. I'm expecting to have perhaps four to six releases during beta. Ideally I'd like to have around one hundred bits of feedback for each of these releases. That would equate to needing as many as three thousand people to sign up. That's a lot of sign ups!

Any tips you an share would be great as I've only got just over twenty sign-ups at the moment and really want to hit at least five hundred before I press the button and send invites out.