This has been hashed to death in other threads, so definitely take a look.
Still, here's my $.02: it's different for everyone. At the very least, you get to bond with some of your classmates (assuming you don't wind up hating them).
If you are lucky, you come out better prepared. If you are unlucky, your study group wastes a lot of time that you could have spent studying on your own. Hard to predict beforehand.
As for how many people, again, totally depends. Organized well, different perspectives can be great. On the other hand, there are obvious distractions.
Give it a shot, see how you feel with it. The one thing I would caution is this: never rely on the others to do your work for you. Don't split up the reading, or the outlining, etc. Going through it yourself will help you learn and prepare for the exams like nothing else will.