[This reply is partly in response to Patty, but my suggestion is similar/related to Jim's.]
Mark, I'm trying to think how to increase profits for you without too much complexity (and with only a tiny increase in support desk calls) but while allowing the potential for large non-commercial groups.
Your proposed pricing structure puts the onus on group owners to manage membership fees, and so you'd be essentially limiting Groups.io to groups with owners who have the technical and managerial expertise to do that. I'm afraid that if Groups.io wants to be able to tap potential income from large non-commercial groups, then Groups.io would have to take care of some of the subscription payment side of things.
Can I assume that on average, the average user is a member of no more than 5 groups? (Yes, some users are members of dozens or even hundreds of groups, but thousands others are a member of just one or two groups.) This means that if you can squeeze just $2.50 per year from those users, it would be the same as having a premium group owner pay $0.05 per member.
I suggest, therefore, that you create two types of users, namely free users who pay nothing and paying users who pay $2.50 per year. Free users can only join groups that have free-user slots left over. Paying users can join an unlimited number of groups without filling up any free-user slots.
Oh, wait, let me explain "free-user slots". All groups have a number of free-user slots. These are slots (i.e. room for members) that are available to free users. Free basic groups have 100 free-user slots. Premium groups have as many free-user slots as the owner had paid for. So if the owner pays e.g. $20 per month for his group, then his group gets 100+400 free-user slots. This means that up to 500 people who are not paying users can join that group, but once the 500 free-user slots have been filled, any additional members would need to be paying users. Paying users can join a group regardless of whether that group has any free-user slots left over (and when a paying user joins a group, it doesn't fill up a free-user slot). Non-paying users (i.e. free users) can only join a group if that group has any free-user slots left over. (Obviously the group owner can still refuse people membership of the group, regardless of whether they are free users or paying users.)
This sounds complicated on paper, but I'm sure you can figure out a way to write it down simply :-)
The advantages of such a system include:
Whether group owners want to add all of this on their home pages is up to them. Group owners who are willing to pay for all members in a premium plan can simply write on their home page that membership is free. Or, they could write "join our group for only $2.50 per year" (without the need to mention that $2.50 actually allows users to join other groups as well). Or they could write "a small membership fee may be required" and then include instructions and a link to Groups.io's paid plan in the notice.
When a free user tries to join a group, the moderator would be told that this is a free user and be told how many free-user slots the group has left. The moderator can then choose to accept the member, or tell them that the group no longer does (or doesn't currently) accept free users (or that the aspiring member doesn't qualify for it), or (as a third option, provided by Groups.io's systems) put the user in a "waiting list"... for when some other free user leaves the group or for when the group owner decides the person has become deserving of a free-user slot.
People who join via mail (and don't register an account at Groups.io) would always be free users (initially) because they don't register an account at Groups.io. Obviously if they try to join a group that has no more free-user slots, the reply notification would explain that to them, and direct them to the Groups.io sign-up page.
When a paid user doesn't renew his subscription, he gets moved to the "waiting list" of the groups that he is a member of, and the moderator gets a notification, and then the moderator can choose to assign a free-user slot to that user, leave him in the waiting list, or remove him from the group. (An idea for the "waiting list" is that people who are on the waiting list are provisional members, i.e. they get read-only access via the web, but can't post and can't receive e-mail.)
See, $2.50 per year is not much, and even if a user wants to join just one group, $2.50 per year is not much. I'll wager that many users of large non-commercial groups would be willing to pay $2.50 per year for their membership, even if they belong to only one group.
Allow me to speculate about Patty's group (and assuming she started this group after the grandfathering cut-off): it may be difficult for her to manage paid subscriptions herself (setting up a system to get payments, keep track of who paid and who didn't, then reconcile the updated payment lists with the e-group's own membership list, send reminders, explain to members why she had to remove them, etc.). However, she could get a free basic group and simply tell everyone that they need to be paid user of Groups.io to join her group (although she will be able to make a few exceptions for deserving people, i.e. her group's 100 free-user slots, which she can allocate however she desires).
(PS. My suggestions about payment plans in previous mails were based on the assumption that only group owners pay.)