If you know Python, you can use the ArcGIS API for Python, to get the creation date of each user.
created = arcgis.gis.User(gis, user).created
(datetime.fromtimestamp(created / 1000).strftime('%Y-%m-%d %H:%M:%S'))
You could create a list of the users in Python, and sort it based on their creation date.
Use Admin Tools for ArcGIS Online
You can export the list of users to csv, then do what you need in Excel.