I wish there was a way to give money to io groups with no stringsUse the "Upgrade ($20) for one month, w/automatic downgrade to Basic after" button. Use it in a test group if you don't want to use it in a real group. It doesn't even have to be your test group.
I have a couple of grandfathered groups I will not make into paidYou won't. Your Basic group's legacy features are determined by its creation date, not by the date of any upgrades or downgrades.
If I collect money to pay for a group, that money would be counted asMy proposed simplified Donation idea avoids the issue: all payments (yours and members') would be directly to Groups.io.