From my experience doing group study for classes, there don't seem to be any major advantages or disadvantages for pairs vs small groups. The most relevant factor is how many eyeballs looking at something, but even that isn't a huge effect. Both are more effective than working alone (as the article concludes).

For a lot of things, getting together IRL looks like it would work best, but the logistics there can be difficult. For people who have Lesswrong meetups nearby, those are an obvious way to potentially coordinate meatspace study groups.