Cobus,
Best practice is not to modify any System or App Store modules. Doing so makes it more difficult to migrate your app to newer Mendix or Module versions.
One approach I’ve used is to create a new module, make a specialization (or maybe more than one specialization) of Administration.Account into your module and then create a Tenant entity which the accounts are associated with. With this structure, you can then restrict access to data using access rules in the database (i.e. a user can only see objects created by someone else in their company, or that is associated to their company, etc.
Hope that helps,
Mike
Great comment Mike. I will give this some deep thought. Thank you.