There was a video posted on Bike Forums just a few weeks ago that showed a group of high school students who had formed a bike bus. From listening to them talk about it I got the sense that what made it work was the joy and camaraderie they derived from commuting to school as a group, so I don't think it's accurate to characterize bike commuters as loners. Some were leaving home early in order to participate. It was pretty informal; it appeared they had just selected a time and place to meet each morning, and that was it. I think that casual approach is the most likely to succeed. Just meet up and go.