I have mixed feelings about this topic. Heaven knows I'm pro-PTO and certainly I believe groups should strive to increase participation. Aiming for 100% participation is wonderful, but my doubts come in as to how far to take that.
For my 4th grader, areas/organizations in which I need to be involved, volunteer my time, help with fundraisers include 1) PTO 2) Baseball 3) Basketball 4) Competetive Swimming 5) Math Pentatholon 6) Soccer.
For my 6th grader, orgs would include: PTO, Girlstart, Competetive Swimming (2 teams), and Band.
These are the organizations I'm expected to participate in because my child attends school or plays a sport. (And I'm not talking about just attending a game - but helping with a fundraiser, working a concession stand, agreeing to serve on an advisory board, etc. Many cases, even if the sport only lasts one season, the key volunteers work all year.)
Even parents who are caring and involved just can't do it all. Every single one of these orgs expects me to attend meetings, "sign up", and help in some way. And I should - but I have to set priorities.
Neither of my children care if I join the PTO. It happens to be a priority with me, but if I didn't, and a note came home asking for a donation or assistance, I'd help where I could. I wouldn't feel like I wasn't an involved parent because I made the choice not to join an voluntary organization.
At our school, we do have dues ($5). We have a membership drive at the beginning, complete with a class contest. We do what we can, but once it's over, we don't really worry about memberships or statistics. We serve all our families; notes go home to everyone, we solicit volunteers from family. After the drive is over, we focus on the individual events and activities and getting the participation we need for those.