Study alone or in a group? It depends on the personal preferences and personality. if you are introvert type of a person, studying alone might work, but in a group help us develop our skills in a way of interacring with different behavior and nationalities that forces us to focus on our goal. Both works well with me.

I always prefer to study alone but after studying I like to review in a group.

I've tried both and each of them has its benefits. I really enjoyed the time when I studied in group. I spoke English a lot, but sometimes I felt a bit intimidated when I made any mistake. As a result, I avoided speaking too much, I used to be a kind of reserved pupil. I feel that I could have learnt much more if had been more fearless. Another thing I considere a problem is the obligation of being there at the same time. Sometimes I had to miss one or another class due to my work, and when I went to the course I felt literally lost in the class. It doesn't happen when you are a self-taught or when you study with a tutor. You organize your studies according to your availability without extra costs or worries about missed contents. You don't even need to leave your home to study and you don't take the risk of having to practice with a boring person just because there is no choice for you. Well, both modalities have their pros and cons, but I personally still prefer studying by myself.