As per this serverfault answer, use the following syntax:

1. Run this:

2. Login to webmin at https://localhost:10000. Expand the group Webmin, then Webmin Users.

3. Create a group call “WebminRootUsers” or the like. Click through and allow that group access to as many Webmin modules as you like.

4. Create new webmin users, and add them to the newly created group.

5. Test that the new user accounts work.

6. sudo vim or sudo gedit /etc/webmin/miniserv.users, the line with root should look like “root:x:0″ (which is the default).