It doesn't matter really whether or not an IUSR account was created automatically when you installed IIS. All you have to do is ensure that the anonymous account (which, although usually is the IUSR account, can be any account), has the right permissions.
In the IIS Manager Snap in, open up the properties for your website, and in the Directory Security tab, click "Edit" on the Anonymous access and authentication control section, then select a user account that has the permissions you want.
BTW, workgroups and domains don't really come into it.
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