Diff and CrewChief, as always, tell it like it is. I hear what you are saying, that you have a system that works, but you really need to look at things from the outside.
If all you are right now is a small group of friends making things happen, that's great!, but it also can easily appear as though your group is a clique'. Having the Bylaws, elections, meetings, agendas, budgets, a fundraising chairperson, a Secretary, meeting minutes, voting on various decisions, a mission staement, a yearly calendar, etc.; will make those parents who are thinking of getting involved more likely to do so...
All of the amazing work that you are presently getting done will still be possible, and liekly even more. Sure it may take a bit of time at the beginning, but once these things are done it will become second nature. At the same time I understand from your post that you already have people doing certain things. Not sure if you have an official "President" or an official "Secretary", but if not I would recommend that you actually hold an election and promote it. More than likely, with the tight group that you already have, the one that is presently performing those duties will "get the job". But by having the offical, promoted election proceess it will prevent the negative thoughts of your group that we often read about on these very message boards.
Lastly you mention that "speaking with several of the parents many don't want the formality of a board or elected offices", and I have to ask why? What is their, and your, reason for not wanting things to be a bit more official? In many ways, not being more organizaed is unfaor to the school community, the staff and to the students. It may be a few years away, but what's going to happen when you all move on? If your group is more officla you'll likely get new blood, which will be the future of your organization.
I do respect what your group is presently accomplishing, but just think about how much more you can do by becoming more structured.
Good luck,
PresidentJim