I want to reference another user in my answer like I do in comments (
@user). How can I do this?
You can reference another user using their name, and optionally by linking to a relevant question or answer that they wrote, or to their profile page.
This isn't the same as the @ reply used in comments, which actually isn't a reference to a user, but a reply to them. See How do comment @replies work? for more information about comment @ replies.
If your intent is to somehow notify a user that your answer is posted, that's not currently possible. The user who asked the question will be notified that an answer to their question was posted, but you can't specify other users to notify.
As far as replying to another answer: if what you're posting is a response to an answer by another user, it should be a comment on that answer (this will notify the user who posted the answer that they have a comment). You should only post an answer if it is, on its own, an answer to the question. Imagine if all other content on the page were gone and only the question and your response were left. If the page wouldn't make sense, or your response would lose meaning, then it is not an answer.
At this time, a feature like that is not available, for several reasons:
That's not the type of behavior we want to encourage on this site - compare to the following real-world scenario:
So, essentially, by singling out an answer or a user, you humiliate them in a public fashion. As we are a community that encourages all helpful answers, and the input we can get, we don't need people becoming upset at other users and leaving the site.