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