Type @ and the first few letters of the person's name, pick them from the list, and then type and send your message.


 

If the list has a few people with the same first part of their display name, after you pick them you'll see their full name displayed in the message to avoid any confusion.


 

By the way, if you want to get everyone’s attention, save yourself from typing everyone’s names. Just type @All to tag everyone in the space.