May I suggest a[nother] way for groups to get money for premium services? I suggest that owners be given the option to have a "Sponsor our group" button on the group's home page. This is basically a donate button, but the donations go directly to Groups.io (i.e. to you).
The system keeps track of the amount of money that was donated via each group. At the end of a billing cycle, the group owner then has the option of combining his payment with some or all of the sponsored money, to buy another year of premium service.
So, for example, if a group managed to get $300 in sponsorship but they need only $220 for next year's premium membership, the owner then uses $220 of that $300, which reduces the available sponsorship to $80. Or, if a group managed to get $100 in sponsorship but needs $220 for next year's premium membership, the group owner can pay $120 out of his own pocket, add the $100's credits, and the group's sponsorship counter goes back to zero. Or, if a group did get $300 in sponsorship, but the owner decides to pay $170 of the $220 out of his own pocket anyway, the group's sponsorship counter drops to $200.
Sponsored money goes to Groups.io directly and it can't be "withdrawn" by the group owner. If a group is shut down and they have sponsorship left over, they lose the money (or: perhaps the owner can select from a list of 5 or 10 charities, who then get a donation from Groups.io for that amount). If a group drops from premium to basic (i.e. they no longer need the sponsorship money), the credits remain in the group's "account" and is not lost (in case the group later decides to go premium again), but the money can't be withdrawn.
The home page can also (at the owner's option) show how much money has been donated (sorry: sponsored) so far, and/or show a target (set by the owner) plus how close the group is to reaching the target. The system need not keep track of *who* donated -- in fact, any member of the public can use the sponsorship button. The group owner is not informed of who donated money -- only that sponsorship was received, and the owner can see the amount in his control panel (and if the owner chose to make this information visible on the group's home page, then members of the public can also see how much has been sponsored so far).
If it's not too hard to implement, it would be nice if group owners could donate their credits to other groups.
And if it's not too hard to implement, you can consider allowing sponsors to enter their e-mail address when donating, and letting the group owner know not just the amount but also the identity of the sponsor.