Jim, Mary, and I are each sitting at our own homes, wondering what to do. Without any communication, each of us happens to look in the paper and see that Source Code is playing nearby. We each leave our home and walk to the theater.
A good description of what happened: "I decided to go see Source Code, and so, by chance, did Jim and Mary."
Jim, Mary, and I are each sitting together at a bar. We are thinking we should head somewhere else, and Mary decides to look in the paper and see what movies are showing. She shows Jim and me the listings and asks, "Have you heard anything about any of these?" Several minutes of discussion ensues. The upshot of it all is that the three of us head off together to see Source Code.
A good description of happened: "We decided to go see Source Code."
That was a group decision. The central tenet of methodological individualism, that only individuals choose, is plainly false.
"Wow, wait a second," you say, "each of those individuals had to reach that decision in his/her own head."
So what? That is akin to objecting to someone saying "He hit the baseball out of the park" by claiming "Baseballs don't move -- only atoms move!"
Of course, for a baseball to be moving, the atoms in it must be moving. Of course, for a group to reach a decision, the individuals in it must be reaching decisions. The first truth does not mean that baseballs don't move, and the second does not mean that groups don't choose.