When creating new content or adding a comment, you can mention (or tag) a specific user on GCconnex. By doing so, the user can be notified that they have been mentioned in a post (see: How do I manage my notification settings).
Simply type the symbol "@" followed by their name (e..g @Kevin) and choose from the drop-down list, the user you would like to mention.
Note: You may need to wait a few seconds once you start typing the name before the drop-down list appears.
