$uname=$_POST['username'];

$sql3="SELECT group_id FROM workgroup WHERE group_id IN (SELECT group_id FROM people WHERE username='mlopez')";
$query3 = $dbh->prepare($sql3);
$query3->execute();
$data = $query3->fetch(PDO::FETCH_ASSOC);
$group_id = $data['group_id'];

<form id="loginForm" method="post" action="<?php echo('welcome.php?user=' . $uname . '&group=' . $group_id);   ?>">