Author list order by last name

Try using these arguments in your WP_Query

$args = array(
'order' => 'ASC',
'orderby' => 'meta_value',
'meta_key' => 'cap-last_name'
);

Still no luck, this is currently what I got in my coauthors_wp_list_authors function:

function coauthors_wp_list_authors( $args = array() ) {
	global $coauthors_plus;
	$defaults = array(
		'optioncount'      => false,
		'show_fullname'    => false,
		'hide_empty'       => true,
		'feed'             => '',
		'feed_image'       => '',
		'feed_type'        => '',
		'echo'             => true,
		'style'            => 'list',
		'html'             => true,
		'number'           => 300, // A sane limit to start to avoid breaking all the things
	);

	$args = wp_parse_args( $args, $defaults );
	$return = '';

	$term_args = array(
			'order' => 'ASC',
			'orderby' => 'meta_value',
			'meta_key' => 'cap-last_name',
			'hide_empty'   => 0,
			'number'       => (int)$args['number'],
		);
	$author_terms = get_terms( $coauthors_plus->coauthor_taxonomy, $term_args );
	$authors = array();
	foreach( $author_terms as $author_term ) {
		// Something's wrong in the state of Denmark
		if ( false === ( $coauthor = $coauthors_plus->get_coauthor_by( 'user_login', $author_term->name ) ) )
			continue;

		$authors[$author_term->name] = $coauthor;

		$authors[$author_term->name]->post_count = $author_term->count;
	}

	foreach ( (array) $authors as $author ) {

		$link = '';

		if ( $args['show_fullname'] && ( $author->first_name && $author->last_name ) )
			$name = "$author->first_name $author->last_name";

		else
			$name = $author->display_name;

		if ( ! $args['html'] ) {
			if ( $author->post_count == 0 ) {
				if ( ! $args['hide_empty'] )
					$return .= $name . ', ';
			} else
				$return .= $name . ', ';

			// No need to go further to process HTML.
			continue;
		}

		if ( ! ( $author->post_count == 0 && $args['hide_empty'] ) && 'list' == $args['style'] )
			$return .= '<section class="author-info">';
		if ( $author->post_count == 0 ) {
			if ( ! $args['hide_empty'] )
				$link = $name;
		} else {
			$link = get_avatar($author->user_email, 100) .'<div class="description"><a href="' . get_author_posts_url( $author->ID, $author->user_nicename ) . '" title="' . esc_attr( sprintf( __("Posts by %s", 'co-authors-plus' ), $name ) ) . '">' . esc_html( $name ) . '</a></br><p class="bio">' . $author->description. '</p><span class="posts"><a href="' . get_author_posts_url( $author->ID, $author->user_nicename ) . '" class="button smaller" title="Browse Author Articles"><strong>'. $author->post_count .'</strong> Articles</a></span>';

			if ( (! empty( $args['feed_image'] ) ) || ( ! empty( $args['feed'] ) ) ) {
				$link .= ' ';
				if ( empty( $args['feed_image'] ) )
					$link .= '(';
				$link .= '<a href="' . get_author_feed_link( $author->ID ) . '"';

				if ( !empty( $args['feed'] ) ) {
					$title = ' title="' . esc_attr( $args['feed'] ) . '"';
					$alt = ' alt="' . esc_attr( $args['feed'] ) . '"';
					$name = $feed;
					$link .= $title;
				}

				$link .= '>';

				if ( ! empty( $args['feed_image'] ) )
					$link .= "<img src=\"" . esc_url( $args['feed_image'] ) . "\" style=\"border: none;\"$alt$title" . ' />';
				else
					$link .= $name;

				$link .= '</a>';

				if ( empty( $args['feed_image'] ) )
					$link .= ')';
			}

			if ( $args['optioncount'] )
				$link .= '('. $author->post_count . ')';

		}

		if ( ! ( $author->post_count == 0 && $args['hide_empty'] ) && 'list' == $args['style'] )
			$return .= $link . '</div></section><hr class="separator">';
		else if ( ! $args['hide_empty'] )
			$return .= $link . ', ';
	}

	$return = trim( $return, ', ' );

	if ( ! $args['echo'] )
		return $return;
	echo $return;
}