I used to ask if it was okay to photograph people and they would say yes probably 95% of the time. In a group, someone would always stare at the camera. I'd say "just do what you were doing - pretend I'm not here." After a little while, they would relax, ignore me and go back to acting naturally; that is when I would make my photos.
That's not the purist approach, but sometimes you need to do that - groups of women or moms with children are usually not to receptive to being photographed without asking and letting them know why you want to photograph them beforehand, depending on the location and circumstances.