The correct usage is "a user."
The choice between using "a" or "an" depends on the sound that follows the article. "A" is used before words that begin with a consonant sound, while "an" is used before words that begin with a vowel sound.
In the case of "user," even though it starts with the letter 'u,' it is pronounced with a consonant sound (/juːzər/). Since the pronunciation of "user" begins with a "yoo" sound, which is a consonant sound, we use "a" before it. Therefore, it is correct to say "a user."