To mention someone in a comment, type @ then begin letters of their name. For example, if you wanted to mention John Doe, write @John Doe.
A list of people will appear and you can select the correct person you want to mention.
This list will display people who:
1.) are in the thread of comments you're on
2.) people you follow/in inner circle
3.) then, everyone else
Remember, you don't need to be following someone to mention them!