When you generate new user an email address will be sent to the user with a link to authenticate and fully register to have access to the application (API / Playback )
The link only will work once as soon the user verify the account.
The link will expire within 24h after being sent to the user's email. If the email has not arrived please have the recipient check their spam/junk folder. On occasion email filtering systems flag these emails as spam.
If the link is expired it will redirect you to the login page instead, for security reason that will happen and not a page saying the link been expired.
If this does occur the group administrator will need to resend the confirmation email. This is done through the Dubber Portal. A screenshot is below from a users profile page.