It's by invitation only. We've extended it in this wave to those with the highest traffic that meet certain criteria (recent activity on the site, minimum HubScore), because those with the highest traffic would presumably see the biggest impact to their total earnings by participating.
With about 1,500 invitations extended, less than 1% of our user base has gotten an invitation, so don't feel left out. More than 99% haven't received an invitation yet. That's just the way betas go - they work with a small subset of the users first to make sure the system is bulletproof, before extending to all users.
But sit tight! We'll be rolling out the Program to everyone by June at the latest.