Xaun Loc,
There is an "independent umpire of abuse complaints" -- the group
owner!
I think Marvin is looking for a mediator for disputes between a member and the group owner.
That wouldn't go over very well with group owners, I imagine, nor do I think any service such as Groups.io wants to be in the position of mediating such disputes. To the greatest extent possible, the service needs to stay out of the content - the cost of having staff to look at and resolve disputes would likely be ruinous.
So that part of Marvin's request I think will go nowhere.
The other part, about having a feature to resolve disputes, would seem to be handled by the +owner email address. The member (or would-be member) can contact the list managers there to plead their case. I'm not sure what more could be offered in terms of a feature.
-- Shal
Shal Farley