Adding a new user and configuring appropriate access to modules.
Modules can be selected under the "Available webmin modules" including custom commands (I have renamed this) You can not select the sub commands until you have created the new user. Once the new user is created you can select custom commands and further sub select commands which can be run and other security settings.
None of the sub commands are saved and new users have full access to all commands under the custom command modual. This includes being able to add new comands