Add the ability for an administrator on ArcGIS Online and Portal for ArcGIS to see who invited a user and when the user was invited.
Speaking for Portal for ArcGIS here, the portal logs can be queried to find out who sent an invite to a user, and when the user was invited.
About portal logs—Portal for ArcGIS (10.6) | ArcGIS Enterprise
I hope this helps with the portal component of this request. Thanks!
Retrieving data ...