Knowing that WP-Sentry is great for limiting posts/pages to specific groups, why not extend the plugin to limit only sections of a post/page? An example would be to use shortcodes, say [group-a], which was created via WP-Sentry, and encap its contents specific to that group.
I foresee WP-Sentry has the ability to do that, and it'll be, perhaps, more powerful and simpler than another plugin - Role Scoper.
Hope that such feature can be incorporated in the next release. Cheers!!