How can I enroll a device to a group enrollment in

I have seen code for individual enrollment but I can't find any for group enrollment. I need to bulk enroll a thousand devices to Azure IOT Hub and was thinking of group enrollment.Any sample code will be appreciated.

It should be possible both with group enrollment and bulk individual enrollments. From the samples related to How to manage device enrollments with Azure Device Provisioning Service SDKs:

Bulk Individual Enrollments

public async Task<List<IndividualEnrollment>> CreateBulkIndividualEnrollmentsAsync()
{
    Console.WriteLine("\nCreating a new set of individualEnrollments...");

    List<IndividualEnrollment> individualEnrollments = new List<IndividualEnrollment>();

    foreach (var item in _registrationIds)
    {
        Attestation attestation = new TpmAttestation(item.Value);

        individualEnrollments.Add(new IndividualEnrollment(item.Key, attestation));
    }

    Console.WriteLine("\nRunning the bulk operation to create the individualEnrollments...");

    BulkEnrollmentOperationResult bulkEnrollmentOperationResult =

    await _provisioningServiceClient.RunBulkEnrollmentOperationAsync(BulkOperationMode.Create, individualEnrollments).ConfigureAwait(false);

    Console.WriteLine("\nResult of the Create bulk enrollment.");

    Console.WriteLine(bulkEnrollmentOperationResult);

    return individualEnrollments;
}

Create Enrollment Group

public async Task CreateEnrollmentGroupAsync()
{
    Console.WriteLine("\nCreating a new enrollmentGroup...");

    Attestation attestation = X509Attestation.CreateFromRootCertificates(_groupIssuerCertificate);

    EnrollmentGroup enrollmentGroup =
        new EnrollmentGroup(
            EnrollmentGroupId,
            attestation);

    Console.WriteLine(enrollmentGroup);

    Console.WriteLine("\nAdding new enrollmentGroup...");

    EnrollmentGroup enrollmentGroupResult =
        await _provisioningServiceClient.CreateOrUpdateEnrollmentGroupAsync(enrollmentGroup).ConfigureAwait(false);

    Console.WriteLine("\nEnrollmentGroup created with success.");

    Console.WriteLine(enrollmentGroupResult);
}

Update

Take a look at the device samples. You don't need to specify the enrollment type (individual/group) when registering the device. the correlation to the defined enrollment in the portal is done using the certificate the device uses when it registers.

Update 2

See Quickstart: Control a device connected to an IoT hub (.NET) to see how to communicate with a device that was already enrolled to IoT Hub

Hope it helps!