foo.js

let AjaxService = {
  addNewMember(memberName, orderId) {
    return $.ajax({
              url: "/group_order_members",
              method: "POST",
              dataType: "json",
              data: {
                group_order_member: {
                  name: memberName,
                  group_order_id: orderId
                }
              }
            });
  }
}

let addNewMemberCallback = (data) => {} // NOOP

// usage
AjaxService
  .addNewMember('foo', 123)
  .then(result => addNewMemberCallBack)
  .catch(failure => console.err(failure));