Sample code to set the PartyList field for Email Activity.
function setContact() { var partlistData = new Array(); partlistData[0] = new Object(); // guid of the record partlistData[0].id = "8B2AD82B-30D6-E511-811F-3863BB356F90"; // name of the record partlistData[0].name = "Hugh Grant"; // entity schema name partlistData[0].entityType = "contact"; Xrm.Page.getAttribute("to").setValue(partlistData) }
Hope it helps..