Again a question whose answer may vary from individual to individual, but I will back group study
My way of seeing a group study is like this:
1 - Divide the course among your self (who are involved in group study), make division based on interests
2 - Start reading different things individually and gain expertise in the part assigned to you.
3 - Now the toughest part, individuals to become teacher of other for the topic he/she prepared. E.g. A,B and C divided course X, Y, and Z among themselves in the same order. A completed X thoroughly, B completed Y and C completed Z. Now A teach X to B and C and allows an open discussion on the subject, same for B and C who teach Y and Z. This have benefit, knowledge and confidence of A increases in topic X, B and C have benefit that they have a very good knowledge of X before they start preparing it. This will save time of B and C when preparing X by more then 50%. An over all win-win situation.
4 - For sure in group studies success depends on how serious you are in your studies.