Talk with your daughter and explain that the sensations she is feeling are good ones and are normal, but that her behavior must be limited to times when she is alone. If she continues to do this around other people, remind her quietly that she should go to her room.
Most children stop stimulating themselves in public as they get older. You may want to talk with your pediatrician at some point to rule out the possibility of an infection or irritation if the behavior seems to increase in frequency or intensity.