Do you support a guest network with captive portal / MAC authentication? If you do, you would be bombared with emails each time a device connects (intentionally or unintentionally). The MAC address will be populated into the endpoints database.
I'm not sure there is a way to setup an alert immediately as a device is added, however, you can setup a report in Insight to be sent every day or create a couple reports that send every few hours of endpoints. Add the "Added At" column to selected, then you can filter the csv file for "added at", which should include date/time stamp.
Not sure that will completely satisfy your request, but should get you closer.
Make sure you check the box for including raw data output - you will receive a PDF overview file and a csv file that includes the details - each individual endpoint.