Group study is a very beneficial. You could quiz each other and fix the holes in their knowledge. I think having things like races and competition makes it more fun. Have penalties and prizes, etc.
Keep it up to a group of around 4 people, and invite people who you know will effectively engage in study. Even inviting one person who doesn't do anything but distracts, will make you do minimal work. Take hourly breaks for around 5-10 minutes though. I also recommend that everyone should eat first, that way you have more time to study. It takes a ridiculous amount of time to travel to eat and once you start talking, it's hard to stop. And when you resume work, it's not like you're in the mood to do it anyway. You can always eat together after.