Lots of people have an opinion on methodological individualism, but no one seems to care very much what it is. A semi-recent paper by Geoffrey Hodgson traces the evolution of the term, and demonstrates pretty convincingly that the term has no fixed meaning at all. Different writers define it differently, and even their clashing definitions tend to be rife with ambiguity.
Some people seem to mean by it nothing more than that only individuals have motives and intentions. (A necessary caveat: The terms motives and intentions here have to be understood as conscious, psychological states. Every member of a group may want to see a play, in which case it would be perfectly good English to say that “the group wants to see the play.” But even if 50 people hold the same intention, there is no single psyche holding it for all of them.) The problem, of course, is that this has nothing at all to do with methodology. It is an ontological claim, not a methodological one; and is completely consistent with taking families or even nation-states as the basic unit of analysis. If a group acts as if it were controlled by a single mind, there is nothing methodologically suspect about pretending it is controlled by a single mind. If we want to determine the influence of the earth’s gravity on the trajectory of a cannon ball, we could, in principle, add up the effect of each individual atom’s gravity. But most of us would rather use the shell theorem, and pretend that all of the earth’s mass is concentrated in its center.