jgrn307 said:

For a given permission group, say user "A" who belongs to his permission group creates a page "X" which is a subpage of page "M".  I would like to restrict user "B" who is the member of the SAME permission group to be able to create a subpage of "M" (e.g. page "Y") but NOT be able to create a subpage of user A's "X" page.

Sure, here's how:

• Ensure that your users do not have the "Edit Others Pages" capability via User Role or Group Role.
• Assign your group a General Role of Page Author.
• Assign your group a Page Associate role on the Page Associate role (but not on its subpages).