Thank you for your question Kars!
Users can be managed and given different privileges. Current we don't have a knowledge base article on it. Here is a link on managing the users.
User Manager
Unfortunately, limiting a user to one article does not appear to be a setting that is available. If there is a specific reason you want to do this, let us know what you are trying to achieve and we can see if there is a work around.
Permissions can be set for users for different levels of access as well. This following article may help as well.
User Options
Regards
James R