Yes, group therapy is effective for anxiety. Why is this so? Well, one of the hallmarks of anxiety is that you use avoidance to deal with your fears and worries. So many people with anxiety would naturally avoid going to groups of people to discuss and problem-solve all about their anxiety. Certainly, talking about your anxiety in front of other anxious people may seem too anxiety-provoking to be worth going, but miraculously, it does work.
Going to group therapy works as it helps you to get exposure to your fears and worries. If you don't expose yourself to your fears, then you just keep avoiding. And avoiding just maintains your overall anxiety, as you never get to find out that the anxiety will go away naturally on its own if you just expose yourself gradually to your fears. In addition to exposure to your fears, group therapy helps members when they hear how others cope with their own anxiety and fears. Also, members in the groups tend to be supportive of one another, as they are all going through the burden of anxiety themselves.