Good question!
We define membership as 1) any parent or legal guardian of a child attending the school, and 2) any paid employee of the school (including full- or part-time teachers, the Admin, the janitorial staff, et. al.). Members are allowed to attend and vote at any meeting.
We also have dues, and these also go to the classrooms in the form of teacher allocations, but we don't keep track on a class-by-class basis - all teachers get the same allocation.
We charge 2 levels of membership which also translates to 2 levels of benefits. Regular memebers pay $10/family and get reduced admission tickets to 2 of the major social events of the year - 1 in Fall and the other in the Spring. Gold memberships cost $30/family, but they get free admission to the same events.
If the benefits listed in the thread above don't get 'em, sometimes a financial benefit will