Membership in organization sites are propagated to sub-organizations of that site. All sub-organizations can access and user resources from parent organization site.
When setting memberships for a regular site, it's possible to set it by organization. All members of that organization will be members. However, all members of sub-organizations from that organization won't, as there is no propagation there.
One would need to associate all sub-organizations, and all the organizations below that, which can be impossible to manage if organizations go deep.
Work with organization sites.
It would be interesting to have an option to allow propagation of membership to all sub-organizations.