You are processing information with others, focusing on the task at hand and you have more opportunities to use more senses. The process of asking the "What if…?", "Why…?" and "How…?" questions and seeking to answer these together challenges you to study at higher levels.
Having a regular meeting time at a regular location with people who are expecting you provides you incentive to stay on top of the material.
Increased pathways to learning
Studying in groups offers more opportunities for new insights and conclusions about the subject area. You benefit from the ideas and questions of others to add to you own.
The group process allows you to see that you are not alone in a difficult area of study. It is affirming when you can share something that you know with someone else.
The views of others will present different ways of processing material and predicting the examination questions. When you have only your own views you might miss studying a view that is perceived by another.