Want to turn most parents off from your group? Badger them with shoulds, like you shouldvolunteer or you should want to help the school or you should care more for the kids.
It's so tempting to use the guilt language -- especially during that week when you've missed three family dinners and stayed up too late due to your volunteering -- but it simply doesn't work. In fact, it has the opposite effect; it turns most parents away.
There's tons we all should do. I should exercise more. I should be nicer. I should make the bed in the morning. But I'm sure not attracted to those who remind me of all my shortcomings, and I sure don't want to hear it from my local PTO or PTA.
The key is to be positive and to provide good reasons for folks to get connected to your group. When things are tough and you're low on volunteers, that's when it's even more important to be positive. It's hard, but it's really the only way to grow connections for the long term.
Read my article called 
“Escaping the Guilt Trap” for more on this subject. It's one of the most important concepts in building involvement, and one of the most frequent mistakes well-intentioned leaders make.