Hi Aurelie,
> especially since you can't trace those credits back to a specific user in our group
Actually, you can. An admin can login to the ArcGIS Online organization > Organization page > View Status > Credits tab.
You can click on an analytics column in the histogram, which should display the "Usage Aggregation by Type" donut chart underneath the histogram. When you select one of the analytics usages - you will see which member consumed those credits.
Hope this helps,