First, studying in a group can be helpful for all members. In my culture, we have a famous sound that said "teach what you want to learn". With this manner, you can help other, if you are good in a field; and also you can find many details. The best students can teach hard parts to worse students, and both groups take the advantages of group working.
Second, In group students can find the solution of exercise faster and more correct. For example, when I had the coarse of C++ in the university, the professor asked us to do a project and learn it for midterm. I couldn't solve it, and asked from three of my classmates, but they couldn't too. We decided to work with together, the project was done just in two days. It was like a miracle. I learned working as a group can solve many problems.
Last but not least, study in a group can learn social skills to students. They will learn to give help of each other without bothering themselves. They learn to respect to others and think about other ideas. This way can boost up knowledge. For instance, when I studied for the exam of philosophy course, I decided to study in a group with my friends. We discussed with each other about different theories which caused our grade improved. From that experience, I learned that I must hear to other ideas and point of view carefully.
In sum, with learning in a group you can help others or give others help. You can solve problems and questions which may will be in the exam. You can learn some social skills which be useful for the future.