In order to mention other users of the application in a discussion message, just press the '+' and insert at least one letter of the name of the user that you want to mention. A list will appear with the end-users that matches the letter you write.

If you want to change the symbol that will recognize that you want to mention users, you need to change the following user line code in the Web Components 'WWP_DiscussionsWC' and 'WWP_DiscussionsOneThreadWC'. The first symbol is the one that you would have to change:

UCMentions.Attach(!'+', &Message.InternalName)