You can build a CSV report of followers for a Group or Space with the Data Export Service. These instructions assume you are familiar with authenticating in DES and using the DES UI. If you aren't, read this article, How To: Use the Data Export Service API Explorer , then follow the instructions below:
There are four filters used to track following and unfollowing Groups or Spaces:
Example 1. Followers of a specific Group
First select the date range you wish to track following activity.
Add an Action filter for ACTIVITY_FOLLOW_SOCIALGROUP and ACTIVITY_UNFOLLOW_SOCIALGROUP
Select Download Place CSV
Create a pivot table