Hi David,
We always request that people wear earth toned or dark colored clothing on charters. Someone out in the open is going to show up, no matter what, but people hiding behind trees or bushes won't show if they are wearing brown, dark green or black jackets, but red, white or yellow will show through between every leaf and are a major pain to clone out. Unfortunately, it seems like there is always at least one person that is clueless and wears bright colors. At least with digital the worst offenders can be cloned out, but back in film days, you were SOL if you were shooting slides.
If there is a location where it is simply impossible for the entire group to get a shot, we do the runby a second time and have people change positions. This also encourages people to take an alternative viewpoint for the other runby and sometimes some great photos come out of that.
Michael Allen