Ideal for my PTA groups would be Basic, with Direct Add "à la carte" at
a lower fee. Something well under $100/yr, that I can get into the
budget, or find a parent to donate, or donate myself.
If you need the direct add for a one time thing, to populate the group, for example, you could pay the $10 for one month, add everyone you need, and then downgrade to free. Would that work for you?
Paid subscription (shopping cart, you split the subscription fee with
the group owner)