Could you explain your requirement in more details. When you say case should be visible to few users, what are those cases and users who should be able to access the cases, How are they related to those cases.
Basically what is the criteria for selecting which users should be able to see which case? Or do you want to show Cases/Assignments tab for few users only ?
You can control Create Case menu using privileges. For the case that you want to hide/show only for certain users, you can provide a privilege in pyStartCase flow's security tab for that case and only provide that privilege to desired access roles.
Let us know if this helps or if you need more information :)