if it were me, I would stop going. If the organizer called and asked why I stopped coming to the group, simply tell her the truth - it's negative and stressful, and I feel shut out. Tell her thanks for the invite, but I don't feel like I belong there.
Before I left I also, as several have said, would take those few aside who seemed helpful and upbeat and get their numbers and discuss a new small group with them, but, don't feel bad if they stay where they are. They might have a loyalty to even the most cranky members.
Maybe you could start your own group, maybe post a notice at your fav fabric shop, or in a local shopper. You could set the agenda and really keep people on track. Also, you could have a set small number of people in the group to keep it controllable. It might be the organizer of your group has just lost cotrol of it and can't find a way to ask the whiners to just shut up.
I left a group where I'd paid dues for a year but we just werent doing anything and one person was really bossy (quilt police!) and I was uncomfortable going. sharet