Here are the steps for exporting users of a group in ArcGIS Online or Portal to CSV.
- Launch Admin Tools for ArcGIS Online
- Navigate to the Users Toolset
- Click on Export Users to CSV
- At the top of the Export Users to CSV dialogue box create a filter by clicking the green circle with the +.
- In the filter, choose "Group is {group name}" and click the green check mark. This will list all users within the specific group.
- Select all users listed and click next to export.
Additionally you can rename the exported CSV file with the name of the group. This way you could run the same steps on a totally different group and keep them organized.
Enjoy