When adding more than 16 users to an organizational AGOL account from a CSV file, there are some issues with pagination. Despite whatever I select for the number of items per page (20, 60, 100), only 16 show up in a seemingly random order. If I click the box for "1-16" it will select everyone on the first page but then on the next page I have to check each record individually. It then expands to tell me that the username already exists in the member list, and I then have to hit the cancel button to move down the list and check the next box.
I have tried this in Chrome and in Edge.