[Gmail-Users] New Member Invites Not Working

I have been having trouble with my invites working. Invitation link
states that it was already used to register an specified account, but
from what I've understood my friends and family
can't login, because of the following message:

"We're sorry. This invitation is not valid. We monitor invitations to
protect our users and to prevent abuse. If you'd like to sign up for
Gmail, you can now do so using your mobile phone. Click here to sign up

<https://www.google.com/accounts/SmsMailSignup1> (US only). Otherwise,
please ask a friend with an account to invite you directly. "

I have tried just about everything listed previously. It seems to be
creating the accounts in Google but not GMail. Any ideas?