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