Mentioning a specific user will allow you to grab someone's attention quickly as they will be notified right away.
Open the chat that includes the user you’d like to mention. It can be either a One to One or Groupchat
In the chat-box, type "@firstname" to stat the search for the user you'd like to mention.
Continue to enter the user's name until the person you wish to mention pops up. Hit the enter key to add the mention to the chat.
Their name will now appear in blue withing the chat. Then enter the rest of your message and send it. This will send a push notification to the user in question.