The sponsor of a guest is marked in the sponsor field that the guest enters on the form. However, if the user from the BCC address approves the guest, then this is not automatically updated. Thinking quickly, the only way I can see this happening is to get the time/date stamp that the visitor account was enabled (should be in the records) and then look at access tracker just before that time. Who has logged into ClearPass? That should lead to your answer.