Currently, the modified date returned from a group in ArcGIS Online, or ArcGIS Enterprise does not change if members are added or removed from the group. It only changes if the details of the group are updated or if the sharing of items to the group changes.
I have a process in place to scan the groups in our org and populate tables of groups, group members, and group items in a spreadsheet. Having the modified date value change with changes in membership would make it easier for me to limit this scan to only groups that have had a change.
Currently, since I don't know if membership changed, I have to scan every group to find any changes.
