Essentially you need a csv file with columns named for each parameter of the create command, how you create the csv is purely up to you. You then redirect the csv file to the create command using substitution to replace the tags with the values from the column from each row in the file. So, something like this, assuming you have a csv file called "newusers.csv"
file: "newusers.csv"
primaryemail, password,firstname,lastname,notifyemail
...
you can create and notify your new users with GAMADV:
> gam redirect csv newusers.csv gam create user "~primaryemail" firstname "~givenname" familyname "~lastname" password "~password" notify "~notifyemail" subject "Welcome" notifypassword "~password" message "Welcome to your new account."
You can also use a template message file, my personal preference, to notify users and have gam fill-in the template tag fields from the csv file.