For exp/spiritbond groups, there are so many good spots you can go to - at least 3 for each 5-rank span - there's no reason to bother others who are already camped. It's downright rude.

I've noticed, though, that simply asking people how much longer they plan on being there works really well. I don't know if it's luck or what, but it always seems as if the group who is in the camp my party wants to use, is leaving in 10 minutes / 10k exp anyway.