When Gmail first came out, it relied on an invite-only system to spur user growth, and it was a success. If you have a product that people desperately want, you can get their attention by playing on their FOMO (fear of missing out).
The email search and management features that are now standard in all email clients were first introduced by Gmail. Due to the success of the invite-only growth hack, people have begun auctioning off Gmail invites on eBay.
However, proceed with caution when employing this so-called growth hack. Google has used this strategy again and again with flopped products (like Buzz, Wave, and the nearly unrecognisable Google+).
This development hack relies heavily on public resources and collaborative efforts in order to be successful. Thanks to Gmail’s compatibility with other email providers, the invitation-only rollout was successful. However, restricting invitations backfired on closed social networks like Buzz and Google+ because the service becomes useless when there are no other users.