Resolution: Authors within TIBCO Nimbus can import users and groups via a .csv file - however they must have the "Account manager" option ticked, within the System roles tab on their user profile.
Go to Tools > Users > User accounts
Double click on the author
Go to settings tab
Select Account Manager and click OK
More information on the format required of the .csv for import can be found within the Help Guide by searching for "Import User Accounts"