SP.User newUser = ctx.Web.EnsureUser("domain\\username");
ctx.Load(newUser);
ctx.ExecuteQuery();

SP.FieldUserValue userValue = new SP.FieldUserValue();
userValue.LookupId = newUser.Id;
listItem["userfield"] = userValue;
listItem.Update();
ctx.ExecuteQuery();

var users = new Array();
users.push(SP.FieldUserValue.fromUser("testAias1"));
users.push(SP.FieldUserValue.fromUser("testAias2"));
item.set_item(columnName, users);
item.update();