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