In 6.0.x this is controlled by this option:
Preferences -> Address Book -> Include addresses in shared address books
I'm not sure if this is available in 5.0.x though as ZD (which is based on 5.0.x) has no such option.
If it is not in the web interface, you can check if it exists on the command line using:
Code:
zmprov ga user@domain.tld | grep zimbraPrefSharedAddrBookAutoCompleteEnabled