See video with attempts to mention users in group:
User Suggestions do not work when sending messages, you have to type the user name in FULL Eg. Typing 'Sanj' does not load a suggestion for Sanjay only when the full name SANJAY is typed the user pops up at the bottom.
Excellent implementation of suggestions is when instructors are adding users to course, see screenshot:
It would be great to have this feature across the site for all users to quickly add friends to comments, Instructor mention students in discussions etc.