To specifically mention someone in a message with whom you are in a group, write the "@" symbol and select the name of the contact. When you mention someone, a notification will be sent to that contact, indicating that they were sent.
Note: Mentioning someone in a group will override any silence notification that the contact has established for the group, unless they have silenced their individual chat together.
If you have been out of a group, you can quickly catch up with the messages in which they mentioned you or answered by touching the "@" button that appears in the lower right corner of the group.