How can a user related to an Organization login and be restricted so they only see related members of the organisation
I will try to answer though the english version of your question may mean i guess wrongly.

if the goal is to have John connected to organisation First Co, and Mary connected to organisation Second Co, then yes it is possible that when John logs in he is restricted to seeing only contacts connected to First Co and will not see those connected only to Second Co., and the opposite will apply for Mary.

EDIT: To do this you need to apply Access Control ACL  and the way to do that will depend a bit on the general purposes of your site.

One option for example is that you can set ACLs to work on Relationships (I can provide more detail on this) so that for example John or anyone else who is an 'officer' of First Co can then access anyone who is an 'employee' of First Co.

To achieve the latter involves installing extensions. In this case the Related Permissions extension and the extension that it depends on, CiviCRM Entity Settings.

When the above extensions are enabled, on any Relationship Type you should now see a new field saying Always Permission A to B (and vv). And at civicrm/admin/setting/misc?reset=1 you should see an option for 'Allow second-degree relationship permissions'.