Group travelers also buy assigned seats more often.

To add on to this, the issue in the article is really more about one-way tickets than single passenger, if you buy a round-trip ticket for that flight the difference in price goes away. In fact, a single passenger round-trip ticket that includes the flight in the article is cheaper than buying the one-way ticket on its own (as in, both tickets _together_ are less than the one-way, it's cheaper to buy the round-trip and skip the return flight). Google suggests that one-way tickets get uniquely screwed because they're often used for business-related travel, but I don't really see anything definitive.

You can also get screwed in the other direction where groups are more expensive - airlines will sometimes bump every ticket in a group to a higher fare level even if they still list one or two tickets at a cheaper price for smaller groups.