Study groups are essential for medical school and premed success, yet this is only true if you pick the right people to study with. Try to study with people who are getting good grades, who are organized, who have a study strategy you wish to emulate, and who won’t be a distraction. It’s better to study alone than to waste time "hanging out" with people over some text books. If you want to go have fun, then go do something fun with your friends!
Studying with other students who are doing poorly will rarely help you. It may seem obvious, but finding someone who is excelling effortlessly and copying their behavior is the easiest way to improve your study habits. If you're doing well and want to spend some time helping out other students, then go for it. Teaching information to others is one of the best ways to see just how well you know the information yourself. If you're doing well it is likely that other study groups will frequently ask you to stop by.
The most important factor to consider is if studying with friends is going to be more distracting than motivating. Even the most efficient study teams you can anticipate an element of socialization. This is not a bad thing. Often, hours of studying can get tedious, and having friends to pass the night with can help with your endurance. Short breaks for conversation can be helpful to recharge and ultimately keep you awake and attentive.
The point of a study group is for everyone to benefit from a group effort. Each person should have a role! For example, you could have each person type up different chapter summery, or have different people make tables or charts to share. You can even have each person prepare a talk on each subject. Sharing material like this is key, as it adds value to the whole group.
Don't forget that you still need to spend time alone studying. You can't only use study groups. There are some things that you just need to take the time to sit down and understand on your own.