There could be quite a number of reasons for this.

1. Are they being sent to a Yahoo!, MSN or Hotmail account?
•There have been sporadic reports from users that the invites are being sent to the spam or bulk mail folders. Check there first.

2. Did you get a bounced message?
•If the message was rejected for some reason, you should receive a bounced message notification.

Another solution:
•Look in your Sent Mail folder, find the invitation, copy and paste the link into a new message and viola!

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