I'm new too, but I think that maybe 1 out of 10 emails actually make it to the inbox of anyone you are inviting with the invite system.  It seems most email services automatically block all GDI emails or put them in a junk folder of the client.  I have tested this for myself with several different email accounts and found this to be true.   I would think a personal email with your GDI link would be a much better way to invite people.