I wrote this utility to make my life easier during implementations and after a few requests I decided to clean and package it up. This is compatible with PostBooks/xTuple 3.8.0 and above, apply using updater 2.2.4 or newer.
After installation you will find a new entry under System > Utilities called Copy Privileges, this requires the MaintainUsers privilege to use.
You will need to select the source user and the destination user, and you have the option to copy privileges and roles individually or at the same time. I definitely recommend clearing the destination users privileges and roles before you copy so check those boxes as well, and then click copy. Once finished you will be presented with a screen display the number of privileges and roles copied.
Please note that this is a package I am releasing on my own and is in no way supported by the xTuple support staff. If you run in to issues, please post here and I will do my best to help you out.
*EDIT* version 0.1.1 attached, which includes fix for issue Alfredo pointed out