Rename this to plugins/globaladdressbook/config.inc.php

'username'
This is the name of the dummy user which holds the global address book.
The username does not have to belong to a valid email account. The username will be stored in the RoundCube database but will not be able to log into RoundCube unless it belongs to a valid email account on your server.
To create a single global address book for everyone who access RoundCube set this options to something like: '[global_addressbook_user]'
To create a global address book per email domain which RoundCube serves set this options to something like: 'lobal_addressbook@%d'
The username can contain the following macros that will be expanded as follows:
    %d is replaced with the domain part of the logged in user's username
    %h is replaced with the imap host (from the session info)