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