joe buissink addressed this topic in one of the sessions he did at creative live. Basically how he handles it is if he sees “Uncle Fred” milling around with his camera he steps aside before the formals and lets them take some shots and then politely asks them to step aside so that he can shoot. Seems like a amicable way to handle the situation. It seems like a polite way to let them feel like they are participating and would make them less likely to interfere after they have had a short opportunity to take some photos and would only take up a minute or two.
