It's badly worded in the article but I think it means person A is entering his pin. Persons B, C, D, etc. take selfies in the vicinity of person A. By compari

It's badly worded in the article but I think it means person A is entering his pin. Persons B, C, D, etc. take selfies in the vicinity of person A. By comparing multiple selfies from sightly different times, you can determine person A's pin.