1. If you are the group leader, you have responsibility. And responsibility is an evil sea monster that swallows you whole and spits out the spare parts, like your dignity.
2. No one has the same schedule. Attempting to get four students in the same room for an hour, that is not normal class time, is about as impossible as trying to force feed a cat medication without the skills of a licensed veterinarian.
3. Inevitably, someone does not know what is going on.
4. No matter how much you prepare, someone is going to go off topic or totally misinterpret their section.
5. Almost always, one of the group members will have a nervous breakdown. And it's usually the leader.