private static void GetContactsByGroupExample(EsendexCredentials credentials)
        {
            var groupService = new GroupService(credentials);

            try
            {
                var collection = groupService.GetGroups(_accountReference, PageIndex, PageSize);
                var contacts = new PagedContactCollection();

                var groupId = "";

                foreach (var item in collection.Groups.Where(item => item.Name == "Test group"))
                {
                    groupId = item.Id.ToString();
                    break;
                }

                if (groupId == "") return;

                contacts = groupService.GetContactsFromGroup(_accountReference, groupId, 1, 15);

                foreach (var item in contacts.Contacts)
                {
                    Console.WriteLine("\tContact Id:{0}\tNumber:{1}", item.Id, item.PhoneNumber);
                }
            }
            catch (WebException ex)
            {
                Console.Write(ex.Message);
            }
        }
        private static void AddContactToGroup(EsendexCredentials credentials)
        {
            var groupService = new GroupService(credentials);
            var contactService = new ContactService(credentials);

            try
            {
                var guid = new Guid("{YOUR Contact GUID}");
                var contact = contactService.GetContact(guid);
                groupService.AddContactToGroup(_accountReference, "{YOUR Group GUID}", contact);

            }
            catch (WebException ex)
            {
                Console.Write(ex.Message);
            }
        }
        private static void GetGroupsExample(EsendexCredentials credentials)
        {
            var groupService = new GroupService(credentials);

            try
            {
                var collection = groupService.GetGroups(_accountReference, PageIndex, PageSize);

                foreach (var item in collection.Groups)
                {
                    Console.WriteLine("\tGroup Id:{0}\tName:{1}", item.Id, item.Name);
                }
            }
            catch (WebException ex)
            {
                Console.Write(ex.Message);
            }
        }