The PIN is the unique identifier on the device but the email address is the unique identifier on the BES. If you did an activation on multiple devices using the same email address, all that will happen is that the last person to do the activation will be OK, while the previous devices will be deactivated.
You will have to have unique email accounts for each device. You can of course simply lock down the devices after the fact (via the IT policy) by not allowing them to use email and PIM functionality. That seems like a waste of a good device though :-)
Craig J. Johnston