As we all know that Google plus is just a copy of Facebook but with new interface and some new features. Like Facebook, Google Plus also let you mention someone (Friend) in comment, status, message etc.
Mentioning someone on Google plus is same as like Facebook. But on Google plus you can use @ or + symbol to mention a friend.
Type @ or + symbol and then type first few letters of first name or last name of friend.
Below images will help you understand easily.