Right. IMO too complicated, both logistically and politically/psychologically. Better for everyone to pay the yearly $2.50 completely unrelated to any specific group. Drew's suggestion.
I was going to suggest that perhaps there should be a plan that includes no free slots, for group owners of your persuasion.
But there's a numeric question that comes into play: In Mark's original Pricing Changes number of included members x the per-member cost equals exactly the base cost of the group (for both Premium and Enterprise, monthly or yearly). Subtracting off the included members would leave a plan cost of zero.
If we exclude Basic and Enterprise groups from this option it may be ok for a type of Premium group to have a zero base cost, or a very nominal one, so long as every member (including the owner) has a paid Groups.io account.