The key to success with a study group is open communication. This will inform everything else you do and make your group that much stronger. For instance, you should go in with an open mind and a desire to find out what everyone else in the group is good at doing. Play to your individual strengths and use those to help the group. Some members might be better at synthesizing the day's lesson and explaining it again to everyone else, while others might have a knack for research that sheds new light on assigned problems. Everyone needs to contribute, but those contributions won't all look the same, and they don't have to.
Further, you should set aside a regular time to meet and make sure that everyone involved does the assigned work. Some fluctuations are totally understandable — college is a busy time for everyone — but if someone isn't chipping in and is just coasting by on the efforts of the group, it might be time to re-evaluate whether you want that person in the group. Study groups should be fun, but they're ultimately designed to help the members come to a better understanding of the course material, and that can't happen unless everyone participates equally. Each week, distribute assignments for the group, whether it's a few sample problems from the text or a specific task like research. When the group reconvenes, everyone can share what they've learned. Peer-assisted learning is a wonderful way to bolster class lectures.
Another important tip: don't be a jerk. Study groups throw together people of often wildly different backgrounds, belief systems, and worldviews, and it's only natural that some head-butting will crop up over the course of the semester. But letting these disagreements turn into personal conflicts is immature and stupid, and it will distract you from the real reason you're in the group, which is to get a better grasp on the material. Somebody else hates your favorite politician? Somebody else makes fun of your favorite movie? Get over it. Be the bigger man or woman, and just let it go. Healthy arguments about the relevant subject matter can be great in terms of reaching a better understanding of the course work, but when those sink to personal attacks, you're lost.
Just remember: be fair, work smart, and don't lose your cool when things get tough. To be honest, those rules probably apply to life in general, not just study groups, but still. Follow them, and you'll see the benefits in your grades right away.