This ultimately calls https://github.com/dimagi/core-hq/blob/73385e08288fde418512ef301bc5a5ca841a4390/corehq/apps/reports/util.py#L96, which has 

    if group:
        # get all the users only in this group and don't bother filtering.
        if not isinstance(group, Group):
            group = Group.get(group)

self.group_name is actually a group name, not a group id, so Group.get will fail.  I think ProjectReportParametersMixin.users wasn't being used by any reports that have a group filter, and I wanted to use it.