Sky-high likelihood of getting distracted
Every time a colleague has a question or feels they just want to vent about something unrelated, your concentration gets broken. If someone feels peckish they will recommend a lunch break for everyone; or the mere mention that they're going to the cafeteria for something will break your concentration anyway. Phones will inevitably ring and beep. There's seldom a decent stretch of uninterrupted study time when you're with other people.
The timetable isn't tailored to you
You can be rushed or held back if the differential in information uptake is great between study group members: some take longer to understand than others. The pace at which the group moves therefore will never be optimized for you. You're either going to be forced to catch up by taking short cuts or hold yourself back to wait for others.
Where you may be familiar with a topic and only require an hour of revision, most of your colleagues could be weaker at the subject and insist on devoting a full day to it. Of course you're free to study something else in this time, but then that defeats the entire purpose of the study group.
The logistics involved
When you're at home you just take your notes to the nearest quiet table and study. Anything needed is at home with you. If you go somewhere to study with a group you need to firstly ensure you pack everything you need and secondly make the trip to wherever the study session is. This logistical effort seems small but there's also the return trip to consider and by the most optimistic estimation the effort to pack up, travel and then eventually return home will take at least 30 minutes, usually 45. All this time could be spent revising an extra topic. Add up that time over just one week and consider what you could get done in that time.
How clever is your group?
Being in a group may increase the odds of somebody knowing something that others don't, but it doesn't preclude the possibility that everyone has the wrong understanding of a topic. The group mustn't just be a bunch of friends deciding to study together, especially (and I don't mean this in a nasty way) if they're all borderline students trying to scrape a pass. For a study group to work its makeup needs to comprise of students with varying abilities in different subjects/sections, without any overall laggards to hold the others back. This in theory sounds fine, but in practice finding yourself in such a group is highly unlikely, especially given the societal pressure you will be under to study with your friends.
There comes a point where being in a group is counterproductive and everyone simply works best in isolation. Teamwork is not always the solution. Universities have tutorial sessions (known as homework in school) where practice examples are looked at and questions can be asked of a qualified tutor. Remember too that most lectures permit questions. Any more interaction than tutorials and lectures I fear is often more of an excuse to socialize than achieve anything fruitful.