Can you clarify what does it mean " This user can't be an administrator.
". I suppose you that you wish the specified role to be excluded from the settings / administration part of Sitefinity. If that is so please make sure to edit the permissions for all pages in order to allow your custom role to edit/create/delete/modify pages. I have created a sample video which displays how you can achieve that with one of the built-in roles - Editors.
the Telerik team
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