Cant invite new members to organization, "Email not registered".

When trying to add guests to my organization I get the following error “Email not registered”, how to solve? Doesnt work for any email.

I am having the same issue. I know the email address is correct. The user can sign onto their Unity account online so it must be valid. But when I try to add the user just like above I get the same error. Anyone know why or how to fix this?