How to loop over OG group members from comment
I found a work-around by implementing all of the desired functions in the custom action provided by my custom module:


/**
 * Implements hook_rules_action_info
 */
function mymodule_rules_action_info()
{
    return array(
        'mymodule_comment_notify_og_members' => array(
            'label' => t( 'Notify OG members of comment' ),
            'group' => t( 'Custom' ),
            'parameter' => array(
                'comment' => array(
                    'label' => t( 'Comment entity' ),
                    'type'  => 'entity',
                ),
            ),
        ),
    );
}

/**
 * Notify relevant OG group members of a new group comment
 *
 * @param   object (entity metadata wrapper)    $comment
 */
function mymodule_comment_notify_og_members( $comment )
{
    //~ dpm( $comment->getPropertyInfo() );

    // Get the nodes we need from the comment entity metadata wrapper
    $node = $comment->node->value();
    $author = $comment->author->value();

    // Get all active group members
    $uids = og_get_group_members_properties( $node, array(), 'members__' . OG_STATE_ACTIVE, 'node' );
    $members = user_load_multiple( $uids );
    foreach( $members as $member )
    {
        if( $member->uid !== $author->uid )
        {
            // We're not notifying the author of his own comment

            $subject = format_username( $author ) . ' commented on ' . check_plain( $node->title );
            $body = '';
            $body .= 'Hi ' . format_username( $member ) . ',';
            $body .= '' . l( format_username( $author ), 'user/' . $author->uid ) . ' just commented on the ' . l( $node->title, 'node/' . $node->nid ) . ' group.';
            $body .= '' . l( 'View the comment', 'node/' . $node->nid, array( 'fragment' => 'comment-' . $comment->cid->value() ) ) ;

            privatemsg_new_thread( array( $member ), $subject, $body, array( 'author' => $author ) );
        }
    }
}