It's a strange kind of ViralMarketing. And I wonder if it works - whatever "works" means.
But it's very clearly a sign of NetoCracy. People are getting access to a new, exclusive, service not by being able to buy it, but by having the right social connections.
People are begging for invites on TribeNet, offering them as GoogleJuice. Some are undoubtedly imploiting them, and others are exploiting them by auctioning OnEBay. (http://search.ebay.com/gmail_W0QQsokeywordredirectZ1QQfromZR8))
They're discussing this, a long way down this otherwise not very exciting TribeNet thread : http://www.tribe.net/tribe/servlet/template/pub%2Ctribes%2CViewThread.vm?threadid=118e0102-be2e-4f9e-a412-c4ef240c955c&tribeid=322ce0e2-5b4d-4bda-8030-78cb93abd866
This struck me : Look at it this way. Google sets the number of invitations. By doing so they effectively set the maximum number of user that can use their system. Considering that they are planning to give people a gig of space that might be a good thing to know. Plus this "exclusive access only" factor makes the service more desirable.
In other words, Google are issuing a FixedCurrency backed by disk-space. :-)
See also :