Hi Grazia,

You can only add existing users to a course "by Email" if the person has verified their email address in Canvas.  However, it's likely that they use their email address to log in, so you can add them "by Login ID" and enter their email address even if it has not been verified.

The students can also re-send their email verification by going to Account > Settings and asking to resend the verification from the upper right corner.