To leave a comment for a specific user or group:
1) Go to an asset's Information Page.
2) In the comments section, type ‘@’ attached to the user’s or group's name. For example: @John, or @marketing.
As you type, a populated list will appear to show available users or groups that match the spelling of the name.
The mentioned user, or the members of the mentioned group will receive an email notification and a notification on their Message Board that can take them directly to that file.