The "super secret" thing to me sounds like the friends are just following the etiquette rule that we are always talking about... "don't talk about something in front of others if they aren't invited." It's only a secret because the small group is trying to keep others from feeling bad about being excluded, so they aren't talking about it in front of others who aren't part of it. Isn't that exactly what we would say? You wouldn't want the small group to be saying, "Oh, yeah, Jen, Kate, Amy, Bryony, and me created a Facebook group for ourselves because it was just so annoying having to private message each other every time we wanted to get together. No, it's no big deal, not for the rest of you guys, we aren't really as close to the rest of you." And yet sometimes a few people become friends within a larger group. It's hard for a group of 15 people to really be close, it can just be a pretty big group.