The fact is that our groups are really "parent groups," not "parent-teacher groups," despite the acronyms. And all the time spent trying to get teachers to join or attend or lamenting that teachers aren't joining or attending is time not spent on doing great parent work for the kids and for the school.
I love when teachers get involved, and it certainly serves a group well to have a couple of teachers, at least, who have the time and interest to connect well with your group so that teachers' and parents' efforts can be aligned. But I'd much rather my school have 20 great teachers who aren't that involved with the PTO than 20 just-OK teachers who attend every PTO event.
The best schools have a bunch of distinct talent pools -- like teachers and administrators and staff specialists and parents -- each working toward the same, connected goals. That's what you're after.
Here are some other resources: