I had this doubt from the start. When I try to tag some people in my comment , their name is suggested to me on the top left. But occasionally this is not the case, they dont get suggested. And in that situation I just type the first name , do they get notified if I do that ?
If you comment on a question the OP is always automatically notified and the OP's name is NOT added as a link in your comment.
The same applies if you add a comment to an answer and attempt to notify the answer's writer.
If you comment on a question or answer and cite a person who is involved in the conversation who is NOT the person who wrote the question or answer then the name should "autocomplete".
You can usually check if a notification has occurred by clicking on the cited name (or shift-click for new tab in Windows). This should take you to the profile of the person converned. If this works they have been notified.
Adding comments from others:
You can only 'ping' one person in a comment. (Memory says that this needs to be at the start of the comment).
It is not possible to 'ping' users who are not already active in the comment thread immediately associated with the post being commented on. [eg in commenting on an answer you cannot ping a person who has commented on the question (and vice versa)].