Sorry for any failures there.
In some case where a user was already invited or has blocked our emails previously, there may be an issue with the rules in place. If you'd like to post the affected email address here, we can check on the status for your group.
Sorry for any confusion there!
Have you tried sending a forgot password link to that user from the GoToMeeting login page?
If that failed to, we can check to see if the email is on a block list due to too many failed attempts...
I've set this topic to private, can you please provide the email address for the user? It's possible they are currently on our 'bounce list' due to, too many failed email deliveries in the past.