I posted a question and later found a comment related to my question in another post, so I'd like to invite the comment-maker to say more in my post. So if I @ that user, will he/she be notified?
You have to make a "Hey @poster" comment on a post they've been on; just reply directly to their comment. you can't "ping" people that are have never touched a question (because it would be annoying and lead to lots of false positives if you were to "@John" and notify all Johns on the site).
Also, @user never works in questions/answers, only comments. Questions/answers aren't directed at specific users.