In 5.7.6 and above, you should be able to use CREATE USER
CREATE USER IF NOT EXISTS 'user'@'localhost' IDENTIFIED BY 'password';
Note that the 5.7.6 method doesn't actually grant any permissions.
If you aren't using a version which has this capability (something below 5.7.6), you can do the following:
GRANT ALL ON `database`.* TO 'user'@'localhost' IDENTIFIED BY 'password';
This will create the user if it doesn't exist
Note, if you are on MySQL 8, the GRANT ALL
method will not create a user.