It is not so much the letter that matters; it primarily depends on the initial sound of the word which is directly preceded by the article. The noun user is phonetically [ˈjuː.zə] or [ˈjuː.zəɹ]. So we see that it starts with [j] which is generally considered to be a consonant sound. Therefore we should say and write a user and not "an user" since we use "a" before a word which begins with a consonantal sound.