maybe I'm alone, but I've always found studying with others to be almost a complete waste of time. there tends to be way too much discussion about pointless things like "Do you think he'll ask this on the test?" or "Hopefully he grades this one more leniently b/c the avg was so low last time." Also, I don't see why you'd want a student to explain things to you when you can go to office hours and have the expert explain it instead. at least the professor can see how hard you're working that way, remember your name, etc. Those things matter. Also, in a study group I feel like being silent b/c I'm working on a problem for 10 minutes is awkward, which clouds my mind and makes it harder for me to concentrate. The solitude of my bedroom, where I can talk things out to myself and take as long as I want without the fear of sounding like a crazy person or seeming rude, is much preferred for me.
That being said, if studying in groups works for you, keep it up. I just felt compelled to rant about my experiences with group studying.