Importing is as easy as uploading a file. Go to the import page:

Dashboard / Members / Import users

From the file selector select your CSV file from your computer, To be safe first run a simulation. This simulation will tell if the emailaddress already exists; the username provided is correct and not conflicting with concrete and that the password is strong enough.

If you have errors you have the option to select "Show only errors", This is a filter on all bad inputs of the CSV, it gives an easy list to get on with corrections.

If you are happy with the simulation the you can go back to the import page, upload the same file again and click import. 

Congratulations, you just populated your website!