Hi,
Here is some example code that work

.
It create a contact group "test" with "john doe" and "xxx" :
Code:
var soapDoc = AjxSoapDoc.create("CreateContactRequest", "urn:zimbraMail");
var cn = soapDoc.set("cn");
var fileAs = soapDoc.set("a", "8:test", cn);
fileAs.setAttribute("n", "fileAs");
var nickname = soapDoc.set("a", "test", cn);
nickname.setAttribute("n", "nickname");
var dlist = soapDoc.set("a", "\"john doe\" <foo@bar.com>, \"xxx\" <xxx@xxx.com>", cn);
dlist.setAttribute("n", "dlist");
var type = soapDoc.set("a", "group", cn);
type.setAttribute("n", "type");
var command = new ZmCsfeCommand();
var response = command.invoke({soapDoc: soapDoc});