You Sent the Wrong Email Address
Delete the user entry from the Team page (shows PENDING) and send the invite to the correct email address.
My User is Unable to Find their Invite
Oddly, invitation emails can be marked as Spam or Junk. Check for an email from firstname.lastname@example.org; it contains the validation link.
In some cases, companies filter emails from new unknown domains for security reasons. Contact your IT team and whitelist our domain.
Invite has Expired
Users are expected to sign up within 14 days of being invited. Otherwise, you must resend the invite from the Team page.
My Issue is Not Listed Here