PHP - Subscribe to Group using Mailchimp API

I followed Drewm/Mailchimp API to integrate may custom form into Mailchimp.

I successfully subscribed the users to my list. But when I include the Subscribe to Group line of codes, the whole function is not working, even the Subscribe to List is not working anymore.

But when I remove the Subscribe to Group, the Subscribe to List is working again.

Here is my code:

include 'MailChimp.php';

$MailChimp = new \Drewm\MailChimp('ab4dc1be0f31933f5f8bcdc82231a47d-us4');
$result = $MailChimp->call('lists/subscribe', array(
    'id'                => '0c043be204',
    'email'             => array('email'=>$email),
    'merge_vars'        => array('FNAME'=>$firstname, 'LNAME'=>$lastname),
    //                     'groupings' => array(
    //                         array( 
    //                             'id' => 19361, 
    //                             'groups' => array($_POST['group'])
    //                         ) 
    //                     )
    // ),
    'double_optin'      => false,
    'update_existing'   => true,
    'replace_interests' => false,
    'send_welcome'      => false,
));

print_r($result);

My HTML form looks like this:

<div class="col-md-6">
    <label>Email Address</label><br>
    <input name="Email" type="email" required="required" id="Email" placeholder="Email">

    <label>First Name</label><br>
    <input name="FirstName" type="text" required="required" id="FirstName" placeholder="First Name">

    <label>Last Name</label><br>
    <input name="LastName" type="text" required="required" id="LastName" placeholder="Last Name">

    <div id="group">
    <label>Areas of Interest</label><br>
    <input name="group[]" type="checkbox" id="Interest1" class="interest"><label>Turf</label><br>
    <input name="group[]" type="checkbox" id="Interest2" class="interest"><label>Landcare</label><br>
    <input name="group[]" type="checkbox" id="Interest3" class="interest"><label>Landscaping</label><br>
    <input name="group[]" type="checkbox" id="Interest4" class="interest"><label>Landscaping Architects</label><br>
    <input name="group[]" type="checkbox" id="Interest5" class="interest"><label>Nurseries</label>
</div>

I don't know what I am missing or what is wrong with the code. Hope someone can help me. Thank you!