Group users reports provides group wise active users count. Report has an option to include users having learner role in group ( excludes group admins and group instructors). Report provides two views flat and hierarchical.
Report Flat View -
In flat view site's all views are listed as a flat list. A download option is present to download report in csv format.
Report Hierarchical View -
In hierarchical view site top level groups are listed and an option 'Sub Group Count' is present for each row to view subgroups of that parent group.
By clicking 'Sub Group Count' for a top level group - you can view sub groups. Breadcrumb shows group hierarchical structure.
Download link - downloads list of subgroups and member count in csv format.
You can further drill down in the group hierarchy.