how to restrict default parent company access to users

Summary:
We have multiple company/Facility setup, and have 8 companies where 1 is default parent company and 7 are according to business requirements , how to restrict default parent company access to users having non administrator role. Our requirement is that user should view only those 7 CO/FAC. Tried creating users in each CO/FAC, it throws error as "Duplicate Entry".

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25D

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