The problem that Mark has, and is endeavouring to solve, is one that is - essentially - common to all (or at least most) internet based operations: how do you charge for - monetise if you will - a service whose unit of supply (for Groups.io that is principally membership or post) costs very little individually (cents or fractions thereof), but in the bulk provided, a lot (kilo or megabucks): and is only practicable or valuable because it is in bulk. And which, often and traditionally, are free.
Groups,io service is primarily about members and posts - they are what I have set up groups for. Everything else (however worthwhile) is an extra (which isn't to say they're not valuable).
And, as a group owner (and I would think this the case for most), they (the number of them) are not really under my control - I can hope for many (in most cases - and this what has been encouraged) or few (occaisionally), but how many is actually down to people deciding to join, or post.
And so - because I would hope (or want to cater for) for 'many' (> 100) members - even if this is unlikely (but I don't want to be saying 'no' to applicants) - a free group is not something I am going to want in future. Ok, maybe excptionally, but not normally, for the long term. And as I am not geared up (or willing to become so) to collect (and account) for large numbers of small payments (even if I thought my (prospective) members would pay them) then possible future change to a 'premium' group is not an option.
A Premium group, for an organisation which already charges (or otherwise has funds), and could budget for the cost (including any increase through growth), certainly possible (but without a sensible free goups option, maybe off the radar), but not otherwise, (and similarly for an Enterprise group, if I'm ever in that league).
Thinking more positively - as to how to solve Mark's problem - possibly some sort of 'Donation' feature could be set up, for members of free groups to donate (direct to groups.io) - preferably not tied to any particular group. With a message every so often, saying something like 'You are a member of so many groups, have made so many posts, and have receieved so many messages since ... which we trust you found useful - while free, and ad-free, and continuing to be so, there is a cost to providing the service, so please make a donation...' . Perhaps frequency dependent on donations. And donation buttons liberally, on group pages, message footers, etc.