We are a new parent group that formed just a few months ago. We decided to be a PTO and not charge dues. With there being no previous parent group before in our middle school, we are openly inviting all members (which is any parent/guardian of a middle school child) to attend & get involved. With only a handful of meetings accomplished so far, only a few parents have come. We have openly welcomed them/asked where their interest lies, etc. and invited them to volunteer and/or come back.
We will always follow our agenda, and have a timer in order to keep to business and not stray for too long. As it's stated in our Bylaws that we do not make school policy and act as communication and voluteer avenues, any issues of school policy are referred to the principal.
Of course, one of the benefits to coming to our board meetings that we advertise is the principal gives us a report of activities at the school and we have the opportunity to bring up any issues of concern or ideas; but ultimately school policy is under the school administration. So far the one or two times items that have come up, have worked out well. As the parent leaves knowing their concerns weren't fully discussed to their satification, but they have an apt. w/ the principal to discuss it further, and we appear as a bridge for the two.
In my past experiences, making a point of inviting all parents, I have a 50/50 chance of getting that person involved and working. Which is odds worth taking.