The invitation process that Google uses for Project Fi is largely unknown. There is a lot of speculation but few facts. Here is what I have determined through my research. The only people I have found (3) that have actually received an invitation (and posted their experience) and started the service, have some common traits:
1. They knew someone at Google and were added to a friends and family list of the Google employee.
2. They chose to purchase a Nexus 6 through Project Fi.
3. Live in a metropolitan area.
From this I am making the assumption that Google is picking new customers who will purchase a Nexus 6 with their Project Fi subscription. More sales, more profits, Google is all about generating revenue! They also want testers who are loyal to Google and live in areas where cell coverage (Sprint/TMobile) is better.
There is also a lot of chatter about the Project Fi sim cards for existing Nexus 6 owners being in short supply or simply not available. There is also the opinion that the sim cards are and have been available, but Google is first, only sending invitations based on the purchase of a new Nexus 6 through Project Fi. They are requiring a completed questionnaire after the initial selection, which asks the question about an existing Nexus 6 or a new purchase through Project Fi.
Of course, most of this is pure speculation. After all, it is Google's project and they can set the standards anyway they choose. Most of us, who are existing Nexus 6 owners, and are interested in Project Fi, will have to wait patiently. The "Golden Rule", he who has the gold, rules, in this case Google.
Hope this helps ...