Your invite should be visible at the very top of the first page in the app. If it doesn't, the most likely cause is that you have reset your account. When this happens, any pending invitations are deleted as well.
Another reason for a missing invite is that the administrator, for whatever reason, have chosen to delete the invitation. Please contact this person directly if you think this might be the case. Accessy can't see why or when an invited was withdrawn.
Naturally, the administrator can send you a new invite if needed.