Momentropist
A person dedicated to maintaining order & efficiency during an important moment at an event, without being obvious, bossy, threatening, or rude. A Momentropist inconspicuously keeps the main subjects (people and/or animals) from wandering away, intentionally or not, from organized events such as: outdoor portrait photo sessions, bachelor parties where the groom is the subject of uncivilized activities, sorority initiation rituals where the initiate is the subject of humiliation, wedding ceremonies where both bride & groom are required to attend.
The photographer panicked when he became aware that 2 men from the group of sheep herders being photographed had somehow vanished before the picture had been taken. He was about to look for them when in the distance he saw 3 men. The man in front, a Momentropist, pulled the leashes of 2 sheep upon which the missing herders were seated.