It is totally depends on this material or subject which you are going to study. If it is a subject like mathematics, accounts or any thing then you need to be fully focused for the study because these types of subject requires full concentration and dedication not for few moment but for maximum time. On the other hand, if the subject is like physics, business, chemistry, history, you need to learn all complete details with the discussion among all which leads to great learning and obviously it will be more memorable and you can not easily forget.
Group Study should be ignored as much as you can because most of the friend first start study then starting gossiping with irrelevant topics which will affect your concentration as well as your study. So, try to study alone as much as possible and if you found difficulty in some topics, don't hesitate to study with friends.