In the Collaborators window, under the "Invitations" section, all pending, accepted, declined, and revoked invitations to the project are shown.
Note that invitations are valid for 30 days. The expiration date is shown in this window. If an invitation has expired, it is possible to resend it.
To revoke an invitation, simply click "Revoke". It is possible to resend the invitation to the same person. The revoked invitation will still be visible for the collaborator in the "Invitations" window on the dashboard.