I think basically an unstable sense of self is taking your clues from around you, other people, instead of asking (and answering) what you want to do or pursue from true knowledge of yourself. So, with your group joining/quitting thing, you get the idea to be social but you don't really engage with other people; instead of being linked to people like part of a chain, you are just attached, like a magnet? If the piece of metal loses its attraction, the magnet falls off.

I have joined organizations, gone to work or school, etc. and been part of a cause and as part of that activity I have become friends with people and they remain my friends even now, long after the activity. If you look at my Facebook page, it is people I actually "know" and 95% of them I have met in real life or have been friends with for 5-10 years online. When I first was on Facebook it was primarily to play the game Mafia Wars and I had over 1000 "friends" :-) but now I have 90ish and don't play games anymore but interact with my friends in more personal ways. My school friends from the 1960s are on there and my work friends from the 70's and 80's as well as online friends from other sites where we are friends both on the other sites and, by our actual selves/names on Facebook. Being friends in a surface way where they come and go depending on the activity is not the same as getting to know a person and letting a person know you such that it is the friendship that is more important than the activity.

If one thinks, "I want to participate in this activity" more often than, "I want to be with and get to know this person" (especially one's Self) one's sense of self is about things or events (I like to go to school, I like to run in marathons, I like to party, I like to read books, I like to volunteer. . .) and a person cannot identify as/with these things over time because they are outside one's self and our interests change. Too, one gets stuck in the I like to party/I like to read books conflict and how they are generally mutually exclusive. I have a good sense of humor and enjoy laughing and making others laugh. That does not change whether I am partying or reading a book. I enjoy making laughing "at" myself and my predicament and I enjoy it when my husband and other family members and friends laugh "at" me (as my husband taught me, he does not laugh at me, he enjoys me) or when I can think of something funny to write here. That is part of who I am and that is stable over time and activity.