@mention someone
An @mention is like a tap on the shoulder — a way to get someone's attention in a channel conversation or a chat.
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.
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!
