We have interesting discussions on the message boards and on Facebook about this very challenge.
We also have a great feature on dealing with cliques in your parent group, and I wrote a column on how even the best groups can fall into the clique trap (including those led by well-meaning volunteers).
If you're feeling left out, my advice is to find a place within your school to find success. I'm guessing your goal is to help make your school a great place for your child and other kids and to get connected. You can do that in so many ways at a school -- even beyond the PTO or PTA. You can help in the school library or take on a new, smallish project that no one else is into. You can work as a room mom or dad. There's really no limit. But the key is to stay connected (because it's so important -- you can't be chased away) and then do really good work positively. Be the example of maturity and openness. Make your domain work really well -- and share credit!
Schools and parent groups do need lots of help, especially from motivated volunteers. While there can certainly be obstacles, great volunteers generally can find a path to help. And it's so worth it.