Here's the text:
Thanks for signing up for an invite to join Sway Preview. We've reserved a spot for you.
Hate to wait? Follow us and tweet why you're excited to use Sway, for a chance at earlier access.
I would give out invites first to people telling the world why they're excited to use my product. It's better for my product, really, what's wrong with it? They don't force you to tweet to get an invite, that would be different (and tactically, but not morally, wrong).