Shared phonebook - how does it work?

can someone tell me how the shared phonebook is working?
example having user a and user b, can user a create a phonebook-group, which contains user a, b, c, d and user b can access the same phonebook group and its contacts?
what if user c and d are no playsms user but will become one later on - how are they identified? by mobile number or by email address?

kind regards,

OK, after a lot of trying I found it own by myself:
The user, who is sharing the phonebook (phonebook-group set to “shared to members”), needs to contain the same ACL as the “members” who should see the group.

nice feature.