i have an end user that has received the invitation email, as well as a direct link to the sign-up website, and has signed-up 3 times.  but the system isnt recognizing him.  ive checked his email address is correct, and hes verified thats the email hes using to sign up, but Zoho Desk wont take him.  

everyone else signs up and is recognized by the system, no problem...

help!