Set PartyList field To in Email in CRM 2015 (and earlier)

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..