By default, it is not possible to download or export a list of members from ArcGIS Online. ArcGIS Online also does not have the ability in its user interface to export user's current usernames and corresponding email addresses within an organization to a CSV file. This is a known limitation.
However, there are tools that may be of interest to users. Users can grab the free version of Admin Tools for ArcGIS Online from ArcGIS Marketplace. The tools allow users to export items, groups, and users to CSV and email other users within ArcGIS Online directly.
There is also another option whereby users can download agoTools from GitHub; a Python package to assist with administering ArcGIS Online organizations.
The agoTools are not supported by Esri.