In the rich text editor:
@Richie
In a comment:
[Richie](https://chowlings.com/user/Richie)
For Names and Avatars:
In the rich text editor:
%Richie
In a comment:
[![Richie's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/Richie) [Richie](https://chowlings.com/user/Richie)