If you’re in a group with someone, you can mention them in a message by typing the “@” symbol and selecting the contact’s name. When you mention someone, a notification will be sent to that contact, letting them know they were mentioned.
Note: Mentioning someone in a group will override any mute notifications that contact has set for the group, unless they've muted your individual chat.
If you've been away from a group, you can quickly catch up on messages you were mentioned in or replied to by tapping the "@" button that appears in the bottom corner of the group.