The sytem.user entity is a default mendix entity which you cannot edit.
In your case it might be a good idea to create an 'account' entity which is an generalization from system.user. You can alter the account entity in every way that you like and link it to the organization. The default Mendix 'Administration' module has such an entity.
See this documentation on multi-tenancy: https://docs.mendix.com/howto/security/set-up-your-first-multi-tenant-app
Adding/altering the System.User entity is not possible: setting user (entity) access can be done using the Account entity, which inherits from System.User
You can check the reference guide for more info on this subject (https://docs.mendix.com/refguide/access-rules) or check the learning modules, of which many touch on this subject
I've downloaded the multi-tenant administration module (https://appstore.home.mendix.com/link/app/80498/) and will explore this...
Are there no sample multi-tenant apps one can learn from?
The documentation (https://docs.mendix.com/howto/security/set-up-your-first-multi-tenant-app) is a list of piece/parts to consider rather than a step-by-step guide.
Any help appreciated, this is a canonical use-case that should be well illustrated/documented.
You can set your after start up microflow from the Project > Settings. See: https://docs.mendix.com/refguide6/project-settings
unfortunately, once I set the startup microflow, there are many errors showing up indicating that components involved no longer exist. This is surprising given this module is from August 2017.