If you're moving a large number of users to another community or looking to copy a list of users from Yammer you can do this very easily with tyGraph for Yammer
- Go to the Group Activity Page
- Filter to your Community, you can use the search at the top of the list to find your community faster
- Click "Switch to List Community Members Below"
- Click the ellipsis (...) at the top right of the table.
- Click Export data and follow the instructions for exporting to excel.
See this for details on exporting to excel. Export data from a Power BI visualization - Power BI | Microsoft Docs