In Organisations with multiple groups, the groups can be set to inherit a common address book (Contact List).
This means all users in the organisation see the same contact list, regardless of which project or group they belong to.
The setting can be controlled with a checkbox in General Settings area of each group.
To do this,
1) Click on Settings Icon
2) Select the Groups Tab
3) Choose the Subgroup
4) Scroll down the page and select the Inherit boxes like "Permissions from parent Group" and
"Address book from Parent Group."
5) Don't forget to click Save.