If you need the direct add for a one time thing, to populate thePossibly. The vast majority of PTA sign-ups are during registration (mid-August) or back-to-school-night (early September), depending on the school. There's usually a trickle throughout the year and sometimes batches related to other membership drives; but those tend to be few enough that using the Invite process with the "late-comer" members might be manageable.
One suggestion in that regard is to be sure that the Invitation mechanism also tracks bounces, and provides a UI for the group moderators to see them. That's something missing at Yahoo, their mechanism covers only regular group messages, not invites nor even moderator notifications. That makes them a real pain in today's world of too-aggressive spam filters (especially challenge filters).
Paid subscription (shopping cart, you split the subscription feeI don't really have a basis for thinking so, it is just something that came up when talking with some people about your prior (per-member) pricing model.
Of course it assumes that the group owner has content that the membership is willing to pay for. This would be a completely different kind of group than mine, most likely with the moderators providing most or all of the content. But at a nominal per-year cost to the member that may be at least as common as a group owner believing that it is worth their own money to put their content out there.
After all, who's benefiting from the service? It goes with the increasing public realization that "if you're not paying you're not the customer: you're the product".
(Caveat: free advice is worth the price. If I really knew anything about pricing in this kind of business that would be my day job.)