My response is so much tied to work, where we have thousands of smart, tough and determined people. You are in a tougher spot - no one is getting paid, everyone is struggling with something, I'm not making light of it at all. I know that group therapy has different dynamics from work, but maybe some of the solutions we use at work to get "unstuck" can also help your group.
At first I thought, HEY, this group needs a facilitator to ensure equal time and a flip chart and a "parking lot" page for off-topic issues -- just like so many meetings at work.
But then I read Perna's message - and maybe the point IS for you to learn how to confront the difficult person and to encourage the other people to claim their time to speak about their stuff. And the point is for the difficult person to realize how the time-hogging behavior impacts the ENTIRE group.
It's the situation-behavior-impact model - http://mindmapblog.files.wordpress.c.../sbi-model.jpg
I hope that your group finds a way to change. If not, I hope you find a way to get something from it anyway.