By default, the visibility of all newly created matters is set to public, meaning that everyone on your team can access them and work on them. However, you can change a matter's visibility to private to restrict access to only the handful of members added to such a matter.
Setting matter visibility to private
To change matter visibility to private:
Go to Matters and open a desired matter
Navigate to the Quick Actions section, tap Set visibility to private, then select Yes in the pop-up window