Mentions let you get someone's attention in a group chat by tagging them directly.
In a group chat, type @ and start typing a name. A short list of people in that chat appears above the message box. Pick a person by clicking, or use the arrow keys and Enter. Their name is inserted and highlighted in your message. Send the message as usual.
Yes. The person you mention receives a notification, for example "{name} mentioned you," or "{name} mentioned you in {group}" in a named group.
No. Mentions work in group chats, not in one-on-one direct messages.
If you mention someone who is set to away, a small note appears as you type, for example "{name} is set to {status}" or "2 people you mentioned are away."