The new feature to mention someone using @ is implemented. Now you can @ people in your notes or in your comments. For example, if you put @leon in your notes or comments, the user leon will be notified of the mentioning and will be able to see where you mentioned him.
For now, on the front UI, we didn’t implement the automatic suggest box when you enter the username to @. The reason is @ is a quite social function, and we are not very sure right now we want to make the site very social and noisy. Learning needs a quiet environment or mind, doesn’t it? So we want to wait a little and see how people like using this feature. Once our thoughts with it mature, we will implement it as it should be.
By all means, you can start mentioning someone by using @.