Someone has to have the power to add people to the group. If somebody adds my grandmother, then suddenly it's a different group. There are some things my grandmother is out of the loop on, such as who I'm dating. If someone added my extremely conservative and Christian young cousin, that would change the dynamics as well, because I can share things with my parents and my sister that my cousin would find extremely distasteful (maybe not personal information, but movie trailers would be a good example.)

I like being in control so I know who I'm broadcasting information to. It's not the same as privacy, but if someone else in my family shares one of my NSFG posts with my grandmother, it's their failure of discretion and not mine. That matters.