In my experience about half of PTOs charge a membership fee. Those that don't usually define mmebers in their bylaws. A common way to do that is language like: "all parents and guardians of students at XYZ school are considered members of the XYZ school PTO as long as they have a child attending the XYZ school."
Tweaks on that would be whether you include teachers and when a parent can vote (after new student attends first day, after student is registered, etc.).
Side note: I think the ability to not charge dues and to consider all parents members is a real advantage of the PTO structure. We spend all this time thinking of ways, dying to get more parents involved... and then we make them pay to get involved??? Seems odd to me.
We are contemplating changing over to PTO. Do most schools collect membership fees as a PTO and if you don't, how is the voting process done? As a PTA, only paid members can vote on issues.