I'm planning a very structured situation where each volunteer must manage only some contacts without seeing others.
My question: is it possible, for each user, to limit access (view) to only contact belonging to its group?
For some scenarios it is better to provide the ACL by using Relationships to 'join the dots', rather than using Groups, and this allows you to then create multi-level ACLs.
If someone (X) has Permissioned Relationship to Org 1 and Org 1 has permission to Org 1.1, and person Y has permission to Org 1.1 and if Org 1.1 has permissioned relationship to Z, then X will see Y and Z Y will see Z Oh, and you don't need to add any ACLs at all, this is taken care of via this Fuzion Extension
May not be quite what you need but may be useful to others who read your ticket and who could switch to having Relationships at the heart of the ACL requirements.
This types of structured permissions are managed by ACL and the following link seems to be relevant to you ur question as well: Using ACLs to manage access to contacts
Hope this helps!