An @mention is like a tap on the shoulder — a way to get someone's attention in a Teams chat or Outlook email.
To get someone's attention in a chat, them. Just type @ before their name and then select them from the menu that appears.
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In the box where you type your message, type @, then type the first few letters of the person’s name. You can also @mention entire teams and channels.
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Select the person. Repeat for as many people as you want to @mention.
Each person you @mention gets a notification in their Activity feed. Check for a red circle next to Activity 
right now to see if someone has @mentioned you!