how can authors to define custom pages?

You can do the following:

<?php

$post = $wp_query->post;
$options = get_option( 'sample_theme_options' );
$authors = explode( ',', $options['sometext'] );

if ( is_author( $authors ) ) {
    include ( TEMPLATEPATH . '/brand.php' );
} else {
    include ( TEMPLATEPATH . '/customauthor.php' );
}

?>

This way, you’re always sure that $authors contains an array.