In that case, you have to resend the invitation link to the onboardee/admin. In order to do it, you have to follow the steps below:
1. In the onboardees/users dashboard type the name or email of the person who got the link expired;
2. On the right side of the name, click on actions -> view;
3. When inside the profile, click on the yellow button: "resend the activation button".
4. Wait for the confirmation and the new link should have been sent to the email.