You cannot refer to a user in his/her own post(question/answer) (because they will be notified anyway)
You can refer only one user in a single post/comment
The owner of the post you're commenting on will always be notified of
your comment. If you are replying to someone else who has previously
commented on the same post, mention their username: @peter and
@PeterSmith will both notify a previous commenter named “Peter Smith”.
It is generally sufficient to mention only the first name of the user
whose comment you are replying to, e.g. @ben or @marc. However you may
need to be more specific if three people named Ben replied in earlier
comments, by adding the first character of the last name, e.g. @benm
or @benc Spaces are not valid in comment reply names, so don't use
@peter smith, always enter it as @peters or @petersmith.
If the user you're replying to has no natural first name and last
name, simply enter enough characters of the name to make it clear who
you are responding to. Three is the minimum, so if you're replying to
Fantastico, enter @fan, @fant, or @fantastic.
You can use the same method to notify any editor of the post, or – if
this is the case – to the ♦ moderator who closed the question.