Guy and Girl- If you have a guy/girl group, at least one of you went into the study session with no intention of actually studying. If that's too dishonorable for you, then watch our for your study partner; he/she is more interested in studying you. (Thanks to G for making this apparent to me).
Friends- Studying with your friends is probably counter-productive. You're going to end up ordering food, talking about work or class, and only spending about 1/10th of the time to actually study. I hope you knew the material well!
Last Minute- If someone asks you to study with them and it's the day of the final, sorry bucko. This person didn't study and you look the nerdiest of the class. It's a fine quality, really! I'm just kidding, you probably don't look nerdy. You just stand out in this persons mind as someone smart and approachable.
Unplanned/Suggested- I've had professors who suggest you meet with a group to review, or put you with a group to study during a review session. I find that these groups tend to be a bit awkward, no one talks, and you either study quietly on your own, or with one other person. I hate these groups. I always feel like we need an icebreaker.
Dictator- Have you ever had that group where one person becomes a self-appointed leader, and takes it a bit too far? You end up being assigned certain parts of the study guide to do, and you feel like you're in another class? Been there, done that. I call these groups the Dictator groups. It's best to get out as soon as you can, to avoid the potential guilt trip later.