ArcGIS Online Organization Group Email and Administration Tools
Contact ArcGIS Online subscription group members and owners by email. Available actions include:
Email members of a selected group
Email all the users of an ArcGIS organization
Email all group owners
This tool has also been extended to all subscription Administrators to assign all organization users to a group.
You can try it live here: http://nwdemo3.esri.com/agoladmin/emailgroups.htm