I have not done this in a long time. But generally speaking the white wire is a ground and when they added the third wire it was also a ground. Both the original second white wire and the new third ground wire attach to the same ground block in the main box. I have seen an electrician pigtail the third outlet post and the metal box to the original second white wire in a two wire system at each box. Back when they added the third wire on outlets they essentially were just grounding the appliance being plugged in and the metal outlet box with the third wire. Having said all that I'm not sure anything I said is code compliant.