CiviCRM Community Forums (archive)

What is shared between the various entities in your structure? Are some contacts in common? Most?

They are various users that share part of the contacts between groups, with more or less complex rules of who can see/modify whom.

Out of the box, you can create groups of users and groups of contact and grant some groups (users) access in read or write to some groups of contact.

If your structure is complex and you need ACL, you might want to implement hooks so you don't have to manage the groups yourself but it's done automatically based on their profiles (eg. country).