When a user is mentioned in a comment or a reply, they will receive an email notification. 

The email notification will be sent immediately if the proof is visible for the user, else it will be mentioned in their proof invitation email when the proof arrives at their step of the workflow.
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When writing your comment or reply type the @ symbol followed by the first few letters of their name or email address. You can also type @all or @everyone.

Select the user from the drop down list.

Click the send icon (or use the shortcut key shift+enter) to add your comment or reply.

Tip: Double-click the outer edge of the comment pane, or click and drag it to resize the pane and view comments in condensed mode.