CORE allows you to tag individual people in comments by typing @ followed by their name. This will trigger an email notification letting them know they were mentioned in a given comment.
Tagging Users Already in a Discussion
Tagging a users who's already in the discussion will just send them an email notification that they were tagged.
Tagging Users Not on the Original Package Discussion
When tagging someone in a Package they were not part of, they will be added as viewer for the whole package and they will be able to comment on it. So they will see the comment they were tagged in, the larger discussion, who is on the package if users have the ability to see one another, and all the files in the package.
To give them additional permissions on the package, such as download, you need to share the package with them with your desired permissions.