The dangers of group therapy revolve around one central issue: the idea of bringing together a group of vulnerable people. They could be emotionally vulnerable, malleable, desperate, or volatile. They could be a danger to other participants or to themselves, or they could be easy prey for others in the group.
While there is a clear potential for chaos in a group therapy setting, many of the dangers of group therapy play out in indirect ways and materialize outside the classroom. Members develop a trust relationship with other vulnerable members, and exploit the relationship in private. This could be in the form of anything from rape to robbery to swindling.
This is not to say that there are not benefits of learning and healing with others, taking comfort in not being alone, and learning from others' mistakes. But, given the potential risks, it's important to have a well-defined standard of care for group therapy providers to provide safe and effective classes. This is what many advocates strive for.
Below are some links to others in the Blogosphere speaking on this issue: