Author, though, makes the classic mistake of equating lower PTA membership with less parent involvement in Iowa, when lower PTA membership really just means lower PTA membership. Odds are that a huge chunk of that decline is groups leaving the PTA to form independent PTOs. When that happens, involvement isn't less; it just has a different name.
Also worth taking a read of the comments section. Good insight there as to what you're up against (and how groups are often perceived) in trying to engage more parents. Is your group viewed as a clique? The answer to that question I actually wrote a column on this clique topic a couple of years back.