Select multiple rows from table using checkboxes

I'm trying to use checkboxes for multiple sql query with SELECT.

I have 3 tables, one table for groups one for users and one to connect the user to different groups called groupreluser. Based on what groups are checked, I want it to print out the phonenumber of each user that are a member of those groups.

The checkboxes are created based on the content of the table groups

$sql = "SELECT * FROM groups";
    $result = $conn->query($sql);

    if($result->num_rows > 0){

            while($row = $result->fetch_assoc()){

                    echo '<input type="checkbox" name="checked[]" value="'.$row['group_id'].'">' 
                    . $row['groupname'] . '</br>';

            }

    }

HTML form for submitting

<form method="POST">

    <input type="submit" name="submit" value="submit">

</form>

Code with the sql query to get the phonenumbers.

if(isset($_POST['submit'])){

    $check = mysqli_real_escape_string($conn, $_POST['checked']);

    if(!empty($check)){

            foreach($check as $sel){

                    $sql = "

                            SELECT
                                    groups.group_id,
                                    groups.groupname,
                                    groupreluser.user_id,
                                    groupreluser.group_id,
                                    users.user_id,
                                    users.name,
                                    users.phone
                            FROM
                                    groupreluser
                            JOIN
                                    groups
                            ON
                                    groups.group_id = groupreluser.group_id
                            JOIN
                                    users
                            ON
                                    users.user_id = groupreluser.user_id
                            WHERE 
                                    groups.group_id = '$sel'

                    ";

                    $res = $conn->query($sql);

                    if($res->num_rows > 0){

                            while($row = $res->fetch_assoc()){

                                    echo $row['phone'] . '</br>';

                            }

                    }
            }
    }
}

It works fine as long as I dont use the checkboxes but adds the group_id manually in the SELECT statement. Any idea of why this isn't working? Am I missing someting? Dont know if this is the best way to do it though... Let me know if this is unclear, and I'll try to explain it better