Custom page type / conditional parent page

I need to configure the site such a way that one group of users could add pages ONLY with one parent page and other group would add ONLY other parent page. The final goal is that one group of users could update and add pages (not posts) to one menu, so that their new pages would look like subpages and would be add automatically as submenus. I have reached some success with Gecka Submenu but I dont know how to make one group of users to be able to chose only one parent page.