During beta we limited the number of invitations because we were still figuring out how much load the servers could take. This led us to an interesting discovery later when we added more: we get more
new players through the invitation system when people have less invites to give out. Our theory is that it gives the invites a sense of value, and people are more likely to send them to their friends and family that they feel would actually use them to sign up.
The current distribution is 5 when you sign up, 5 more if you get a little further into the game, and (I think) 5 more when you reach the end of the storyline. If anyone thinks they have more than 15 friends they would like to invite to the game just ask and we will happily add more to your account. You can also just link them to www.extrasolar.com
now that signups are open to everyone.