To mention one or several people in a message, add the "@" symbol before their username(s). If you would like to make it private, be sure to select "In private" before you click "Share".
Once shared, a little lock symbol confirms that the message is only visible to those that you have mentioned.
Groups can also be used to quickly mention several people at a time. Simply use the format “@@nameofthegroup”. You can ask your sphere administrator to create any groups you need.