Passed variable gets undefined variable error on insert on next page

I am passing a variable with $_POST to another page and on the destination page I want to perform an insert operation and want to use that passed value . The variable “$mygroup” is coming from a forms hidden type field from previous page. I am getting below error on insert operation.

Error Message on insert

Notice: Undefined index: mygroup in D:xamphtdocswordpresswp-contentthemesjupiterfunctions.php on line 455

Form Code (to display my form and here the fetched value from previous page display in the textbox $_POST['mygroup'] … but on insert I get the above error

add_shortcode('insertform','insertform');
    function insertform()
    {
    ob_start();
    global $wpdb;
    global $form;
    global $mygroup;
    $mygroup = $_POST['mygroup'];
    echo $mygroup;
    // fetch all the group names for the drop down
    $sql = "SELECT groupname FROM groups WHERE groupname !=''";
    $results = $wpdb->get_results($sql) or die(mysql_error());

    $form.='<style>form label {margin-bottom:15px;}</style>';

    $form.='<form action="" method="post">';
    $form.='<label>FirstName: </label><input style="height: 30px; margin-left: 10px;" name="firstname" type="text" placeholder="Enter Your First Name" /><br>';
    $form.='<label>Last Name: </label><input style="height: 30px; margin-left: 10px;" name="lastname" type="text" placeholder="Enter Your Last Name" /><br>';
    $form.='<label style="margin-right:30px;">Email: </label><input style="height: 30px; margin-left: 10px;" name="email" type="text" placeholder="Enter Your email" /><br>';
    $form.='<label>Date of Registration</label><input style="height: 30px; margin-left: 10px;" name="timestamp" type="date" placeholder="Record the date of registeration" /><br>';
    $form .= "<label>Groups</label><input style='height: 30px; margin-left: 10px;' name='Group' value='$mygroup' type='text' placeholder='Enter Your email' /><br>";
    $form.='<label><input name="usersubmit" type="submit" value="Submit" /></label><br>';
    $form.='</form> ';
    echo $form;
    $string=ob_get_contents();
    ob_end_clean(); 
    return $string;
    }

Shortcode for insert operation

add_shortcode('insertuser','insertuser');
    function insertuser ()
    {
     if(isset($_POST['usersubmit'])){
    ob_start(); 
    global $wpdb;
    $firstname = $_POST['firstname'];
    $lastname = $_POST['lastname'];
    $email=$_POST['email'];
    $timestamp=$_POST['timestamp'];
    $group = $_POST['Group'];

    if($wpdb->insert(
        'members',
        array(
                'firstname' => $firstname,
                'lastname' => $lastname,
                'email' => $email,
                'timestamp' => $timestamp,
                'groupname' => $group
            )
) == false) wp_die('New user registration successful'); else echo 'New user registration failed<p />';
    $string=ob_get_contents();
    ob_end_clean(); 
    return $string;
     } // end of if function
    }   //end of main function