If you believe that an invitation email was sent to you, but you haven't received it, check your spam or junk folder. Alternatively, contact the person who invited you to confirm that it's the right email address.
For security reasons, the "Accept Invitation" link contained in the email expires after 6 hours. If you didn't click on the link before it expired, you cannot revert this decision and accept at a later time. A new invitation must be sent by your IT service provider. Once you get the email, use the link to finish creating your account and set your password.