1) There will be differing levels of seriousness among your friends.
2) There is a risk that your study sessions could deviate into socializing instead of working.
3) Not everyone will pull his or her weight equally.
4) Egos may get in the way and interfere with overall progress.
5) People may not stick with the study group long enough to get the desired results and to justify the time it took to start the group in the first place.