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.