How to assign permissions to a single publishing page

As I know that a publishing page is a securable object, and in my case I need to grant certain users the permissions to Read certain pages in my publishing site (they cannot read other pages), in addition to the users that are inherited from parent site . To do so I did the following:-

1. I edit the page I need to grant the permission to.
2. stop inheriting the permission.
3. But I cannot find a button to create new group to specify the users that can see this document only?

Second point I am wondering about , why I have to stop the permissions inheritance to be able to modify the permissions>. In my case I still want to have the same security setting inherited from the parent site (and any updates made to it) , but I need also to add certain users to be able to read certain pages within this site in addition to the inherited users from parent site.. Best Regards