One error message you might receive when adding users by spreadsheet is Error at line X: Manager could not be assigned. No account found for email address "firstname.lastname@example.org"
What this means is that, while you have assigned this email address for a given employee's manager, there was no line in the spreadsheet for that manager.
In the example above, you will notice that it mentions email@example.com and firstname.lastname@example.org do not have an account. We will need to add a line for them in the spreadsheet and include their first name, last name, and email address.
Once we have created lines for both Sean and Greg, the system will allow the user upload to proceed.