I encounter this countless time..Let me give an insight, If you are tasked to do the job, you want everyone in the photograph to look at you, the photographer.
now some other guests points the camera and the people posing gets confused. Where to look?
When the Official Photographers takes the shot and another one, it ends up some of the posed guests are not looking at the photographer, who gets the blame? The extra "photographers", No its the main Photographer.
Please do not be selfish, you think taking photos are your mantra and right? Please just go and enjoy the wedding as a guest.
Also my clients have known to want us to correct the eyeballs, How?
No wants would like to offend anyone if possible but want you dun spare a thought for others, at least think what your actions can cause you.
usually I rather ok with someone shoot beside me.. if they know how to be "automatic"
let me share with everyone the way I handle this...
during taking the group photos, if I saw some people in the photo look somewhere else because of other "photographers" rise the camera at the same time, I just simply tell the group nicely that I have to retake cos someone not looking at my camera..
if other "photographers" still not "automatic" and try to take again, I will keep retake and retake and retake till someone go and tell those photographers to stop taking photos.
I need not to rise my voice, I need not to give someone stern face....
and I won't see anymore cameras around me thru out the day.....