To synchronize users from a CSV file, you have to create a new CSV synchronization on the User Synchronization settings tab and on the synchronization properties panel, add the source file and set properties of the synchronization.
After the synchronization is set up, you can either manually run it on the User Synchronization settings tab or set it as a regular task on the Task Scheduler settings tab.
Creating a new CSV synchronization
CSV synchronization setup options
CSV file syntax