Very often at weddings while I am making formal portrait photographs of groups of people, friends or family want to stand next to me or off to the side to try and get the same photo I am taking. I am usually on a tight schedule as these photos are often taken just before the wedding ceremony is scheduled to begin and it is hard to stop and explain why I do not want them around with their cameras.
Some people have told me they think it is because I am afraid of losing revenue from print sales or that I am feeling jealous. It is not that. The reason is I want everyone in my photo to be looking in the same direction. I want them looking at my lens.
A group photo works best when every set of eyes is looking right at the viewer of the photo. When there are several cameras present, some eyes will be looking at one of the other cameras instead of mine. Nothing destroys a good group photo more than if some eyes are looking off left, some off right and some at the lens.
I always try to politely ask them to wait until I have finished my shot first. And I always make multiple shots to insure that there are no half closed eyes caught as people blink. When I am done they can rush in and take their turn.
They should remember I am being paid to do my job for the bride and groom, and if they really need the photo, they can get a copy of it from the wedding couple later.