1. Required fields when inserting Users:

• Alias
• Username
• Email
• First Name
• Last Name
• Locale (LOCALESIDKEY)
• Language (LANGUAGELOCALEKEY)
• Email Encoding (EMAILENCODINGKEY)
• Time Zone (TIMEZONESIDKEY)
• Currency (CURRENCYISOCODE) - only required if there are multiple currencies defined
• ProfileId (not profile name but rather the 15-character ID from the URL of the profile in the UI)

2. Pick Lists: The following UI picklist values are different than the values to use in the Data Loader. Instead, use the tabbed worksheets attached (InsertUsers.xls) to review the proper picklist values for the following categories:

• Email Encoding Key
• Locale
• Language
• TimeZone
• Currency

         Scroll down to the bottom of this article to locate the Attachments section.
 

3. IsActive: If you don't specify the IsActive field when inserting a user, the user will automatically be inserted as Active. However, if you do not have a license ready then you can set IsActive to FALSE in your CSV file and activate the user when a license becomes available.

4. Generate new password and notify user immediately: This checkbox is not present in the Data Loader, so a password is not generated with Users created this way. Instead, use the "Reset Password" button and select the desired users from a User list view and reset the password for all selected users from the view itself.

5. Perform insert option with Data Loader:

• Select Insert operation and select User as an object. Browse the .CSV file that you prepared for import and click Next.
• Map the "User" object fields and columns in .CSV file.
• Select a directory to save the "error and success" files and click Finish.

Notes:

• When importing Portal Users, a welcome email is automatically sent which contains a link prompting the user to set a password (no password reset required). For more, please review Import Customer Portal Users.
• License for the User would be assigned automatically based on the Profile selected for the User.
• The description of all the User fields can be found in User.

See also
Prepare a CSV file for an import or update in Salesforce
Install Data Loader on Windows
Install Data Loader on macOS
User (SOAP API Developer Guide) 
Insert, Update, or Delete Data Using Data Loader
Considerations for Inserting Users in Bulk