So weird. I did finally get the invitation email, and went through the process of inputting information etc., and 
I was sent an email with a verification email - which I received!!!!
However, there IS an issue with QB's email system, since  I also wouldn't receive email notifications from Data Protect regarding successful/ incomplete backups etc. The funny thing is that the email address I used today to set up the authorized user account, is the original email address I used with my (main) QB account, until I encountered the problem of not getting emails from QB, at which time I changed (my main email address) it to the current one. And now the current one gave me issues, but the "old" one that I used to set up authorized user account today, can suddenly receive QB emails...Weird. Just weird.