Unfortunately, you can't successfully notify people with @ in comments under a post unless they have interacted with the post in specific ways:
- If they have ever commented on the post, you will be able to tab-complete their username (start typing @name and it will appear) and the tagging will send them a notification.
- If they have ever edited the post, you won't be able to tab-complete the name, but if you put @username they will get a notification (note that this doesn't work for users who suggested an edit to the post if the edit was not applied)
So, if you @ someone in comments under a post they haven't commented on or edited, they'll probably never know about it.
If you think a particular user might be interested in answering a question, you could let them know in chat, but if they haven't been there (if you can't tab-complete their username when you start typing @user) then they won't get a notification there either. I guess most users would probably be ok with you leaving a comment on one of their own posts linking to the post you think they'll be interested in, though you might want to check with them first (I would be ok with it, but there aren't many questions I can answer - also I'm always in chat and happy to be pinged there :).