Well, it's a matter of word usage. I think of "in-group" as a group you identify yourself with, by definition. If someone comes up to you and asks whether you belong to group X, if you're not willing to answer "Sure!", that's not your in-group.
Of course the relationship of an individual to a group is more complicated than a single bit of belong / do-not-belong and one can draw the in-group boundary at different levels of affiliation.
Sometimes our minds suddenly "click" and we see a topic in a new light. Or sometimes we think we understand an idea, think it's stupid and ignore attempts to explain it ("yeah, I already know that"), until we suddenly realize that our understanding was wrong.
This kind of insight is supposedly hard to transmit, but it might be worth a try!
So, what kind of important and valuable insights do you wish you had earlier? Could you try to explain briefly what led to the insight, in a way that might help others get it?