Is it possible to add a small icon to the left of the username, for certain user groups? And if so, what would the code look like? I'm trying to edit Members -> User Groups -> Username CSS
You could use username CSS, but that would make the image only appear in areas where the rich username is used. If you want it permanently, you should probably overwrite the username helper and prepend an image.
It's a problem at your end, the code works totally fine and is unrelatd to your XenForo version. Go to Users->User Groups->List User Groups, select your desired User Group and add
background: url("yourimage") no-repeat scroll left center transparent;
padding-left: 17px;