The functions available to a person with whom you share a PayBook depend on the role you assign to the person.
The Guest role means the person can see the PayBook and its content but not modify it. The User or Admin roles means users can make changes to the content.
Users can also add transactions to shared PayBooks, even if they aren't the Owner and didn't create it, even if it's only available on the "Sharing" tab in your Glass account.
Users may also add labels to the transactions of other users with access to that PayBook