...
Re-login with root credentials
Expand spoiler Webmin and click Webmin Users module link
Click the user name whom we would like to add access to additional module (webadmin, in our example)
Expand spoiler Available Webmin modules and mark the module we need (e.g. Webmin users)
Click Save button at the bottom of the list
Re-login with webadmin user crdentials credentials and check that new module is shown in the list
Done.