Hide Other Account and User Details For Each User.

I Have 2 Accounts XCompany and YCompany. I need to Hide Details of Other Users in Ycomapny and Ycompany itself for a User in XCompany (no accounts should be visiblie to other accounts ) . Now Everything is shown to all users. Including Companies also. I have already tried using Security Groups. But It is also Not Working, means All Companies and Users are seen by user. How Can it Be Possible. May I Get a Step By Step Tutorial to Achieve This

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