Have you added import to the source file?
#import <AddressBook/AddressBook.h>
#import <AddressBookUI/AddressBookUI.h>
Edit: I carefully read your question. There is no ABPersonRef type (at least not publicly available) in the iPhone SDK. You must work with the type ABRecordRef, which is common for both person records and the group - as you can see, all AB *** methods work with the type ABRecordRef.
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