Multi Select Look up in MS CRM 2011

First create three text fields in Opportunity as new_sguid, new_stype, new_sname

Add the below code in the Web Resource

function onchange() {

    var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();

    if (IsNull(originatingLeadValue) || originatingLeadValue[0].type == "4") {

        // Lookup is null or there is a Lead, need to clear hidden fields. To clear one of them will be enough, we will be check this field during onload.

        Xrm.Page.getAttribute("new_sguid").setValue(null);

    }

    else {

        // Lookup contains Contact or User, so need to store lookup properties to our hidden fields.

        Xrm.Page.getAttribute("new_stype").setValue(originatingLeadValue[0].type);

        Xrm.Page.getAttribute("new_sguid").setValue(originatingLeadValue[0].id);

        Xrm.Page.getAttribute("new_sname").setValue(originatingLeadValue[0].name);

    }

}

function onsave() {

    var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();

    if (!IsNull(originatingLeadValue) && originatingLeadValue[0].type != '4') {

        Xrm.Page.getAttribute("originatingleadid").setValue(null);

    }

}

function onload() {

    debugger;

    lookuptypeIcons = '/_imgs/ico_16_1.gif:/_imgs/ico_16_2.gif:/_imgs/ico_16_4.gif:/_imgs/ico_16_8.gif:/_imgs/ico_16_9.gif';//add the custom entity type gif image

    lookuptypenames = 'account:1:Account,contact:2:Contact,lead:4:Lead,systemuser:8:User,team:3:Team';

    lookuptypes = '1,2,4,8,9';// Here first one is entity name, the second one we can get by opening any record for our custom entity and check out the etc value in the url and the third one is the display name for your entity. And i used /_imgs/ico_16_customEntity.gif for the icons to work with.

    var savedId = Xrm.Page.getAttribute("new_sguid").getValue();

    var savedType = Xrm.Page.getAttribute("new_stype").getValue();

    var savedName = Xrm.Page.getAttribute("new_sname").getValue();

    var savedEntityName = savedType == "8" ? "systemuser" : "contact";

    document.getElementById("originatingleadid").setAttribute("lookuptypes", lookuptypes);

    document.getElementById("originatingleadid").setAttribute("lookuptypenames", lookuptypenames);

    document.getElementById("originatingleadid").setAttribute("lookuptypeIcons", lookuptypeIcons);

    document.getElementById("originatingleadid").setAttribute("defaulttype", "4"); // default type - Lead entity

    var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();

    if (IsNull(originatingLeadValue) && !IsNull(savedId)) {

        var value = new Array();

        value[0] = new Object();

        value[0].displayClass = "ms-crm-Lookup-Item";

        value[0].keyValues = new Object();

        value[0].values = new Object();

        value[0].onclick = "openlui()";

        value[0].id = savedId;

        value[0].entityType = savedEntityName;

        value[0].typename = savedEntityName;

        value[0].name = savedName;

        value[0].type = savedType;

        Xrm.Page.getAttribute("originatingleadid").setValue(value);

    }

}

call the onload function on Opportunity onload, onsave function on Opportunity onsave, onchange on Opportunity originating lead change :)