When a new account has been created the registration link to complete the sign-up is only valid for 96 hours. As an admin in your company you are able to trigger a new link for the user in case that happens:
Go to Company > Users and you will see which user still has a registration pending and click on the edit icon on the right
Under Personal info and in the section Login you will see a blue box where you can press the button "Resend registration link to: email"
A new email should now be triggered. Sometimes it may take up to 10 minutes for the email to appear in the mailbox.