Registration link not accepting email addresses

When two of my meeting guests try to register and enter their email address, they get a message that says, "This meeting is for authorized registrants only. Please enter another email address." Everyone else has been able to register with their email addresses so far.

Is there any way to add the two people whose email addresses the system is bouncing myself? I tried to register them using the same link I gave everyone, and I got the same error message.

Thanks for any advice!