Adding users (emails with no zoho account) to a project will not allow them to create an account

Adding users (emails with no zoho account) to a project will not allow them to create an account

Hello,

I have added new users (with no Zoho account) to my project. (refer to image 1)
then I have invited them using (get more Free Project) (refer to image 2)

Now we have an issue:
when they tried to create an Zoho project account with their email it said: 
"an account already exists for this email" (refer to (error-Zoho.png)

Note: they do not have accounts, and they can no longer create accounts with their email.

Please help