Well the good news is that Google has since enabled a feature in Inbox that allows invited users to send an additional 3 invites to their friends or family who might want to check out the service. This is similar to how Gmail started out and how it spread – through the use of invites. This will no doubt curb the scalpers on eBay who are currently selling their invites for an otherwise free service.
Now some have speculated that the reason for the invites is because Google is trying to generate the hype, excitement, and exclusivity that was felt when Gmail was introduced to the world. After all this is Google which means that they should be more than capable of opening its doors to the world if they really wanted to.
That being said, Google mentioned yesterday that a second wave of Google Inbox invites were being sent out, so if you have requested your invite, check your email to see if you might have received it. Otherwise you can try your luck and see if any of your friends have spare invites that they could send your way.