As with all imports from Excel you should always import the data first in temp objects. Because then you are able to produce such an overview. So import the data to those temp objects and then trigger a microflow that checks if that user belongs to one to these three categories. So retrieve first a list of all the users currently in the system. If the temp object has a user that is on that list remove that person from the list. If the temp object has a user that is not on the list it is a new user. And after you have processed all the data the users that are not in the Excel is the remaining all user list.