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.