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If you're really desperate, you can probably find a spare invite on eBay, or, thanks to a new development, you can just ask a friend to share the Google Voice love with you.
As of today, Google is announcing that existing users will be able to invite friends via an "Invite a friend" link on the left-hand side of their inbox. Every user will start out with 3 invites that they can pass along to friends, family, or anyone else, with the ultimate purpose being a shared Google Voice experience that you can really put to the test.
Invitations won't be doled out immediately to all existing users, so don't fret if you don't see the new invite link today. It's a gradual rollout and you'll get your invites in due time.