You can add users in the Customer Administration Portal one at a time, or as a bulk upload. With the Import Users section, you can also change extensions and make basic updates to established users.
Requirements: Customer Administration Portal Admin Access is required for this operation, you also must be able to modify and save CSV files
Navigate to the Administration and Access widget of the Customer Administration Portal, found in the Administration section.
Press the Import Users button along the top ribbon.
Download the reference CSV file.
Edit the document in Excel or another CSV editor.
Save the file.
Upload using the same dialog box used for download.
You can change the extensions for the established users with the same file.
With the CSV file you downloaded, place the new extension in the Update Extension column for any users you wish to change.
Upload as above.
For a longer discussion on this topic, here is this video: