With the right people it works. A few of my friends and I have started a study group for Modern and Ancient History this year, and is definitely beneficial. Allows us to informally go through definitions, essays, important points, quotes, arguments, source analysis skills in a pretty lax environment - no nagging teachers. However, there was an attempt to create one for English which failed after 5 minutes.
Usually it depends on the individual to how effective group study can be in terms of learning and revising. For some group study is vital to remember content via the means of discussion, verbal and aural learning. However, for many people it becomes a bit of a social. If you have the right type of people involved study groups can be quite successful, if you all have the same aims, ideas and goals of the group.
Best advice, try it out, see how effective it is. If you find you're not gaining as much as you would via private study, then opt for a private method of study. If it works, then utilise a group study method in conjunction with private study to maximise revision for the HSC.