The Google Voice service had been operating as a closed invitation only service. Users who joined were provided with a free phone number from which they could direct all incoming calls to any of their regular numbers. That phone number also provided voice mail where the messages were transcribed and also received (and reply to) SMS text messages. Starting today Google is giving existing Voice users three invitations so that they can invite their friends to try the service. When you log into your account the invitations will be listed in the lower left.