Display posts by alphabetical order in a way that first letter of each name is presented.

attendee-list.php

<?php

/**
 * Display 'attendee' post type in alphabetical order by it's category.
 *
 * The output looks like this:
 *
 * 		A
 *		Ace Bill
 *		Alb Nick
 *
 *		B
 *		Bell Evin
 *
 *		C
 *		Carllson Carla
 *		Cure Sam
 * 
 * @since  1.0.0
 * @author Done by help of Justin Tadlock. themehybrid.com.
 * @param  array $atts The attributes to pass to the shortcode
 * @param  string $content The content of the shortcode
 * @return string $attendees_list The data to return for the shortcode
 */
function prefix_features_attendees_shortcode( $atts, $content = null ) {
	
	$atts = shortcode_atts( array(
		'category' => null,
	), $atts );
	
	// Bail if there is no attendee category.
	if ( null === $atts['category'] ) {
		return;
	}
	
	// Set sorted array.
	$sorted = array();
	
	// Set args for getting attendees list.
	$attendees_args = array(
	'numberposts' => 500,
	'tax_query'   => array(
		array(
			'taxonomy' => 'attendee-category',
			'field'    => 'slug',
			'terms'    => esc_attr( $atts['category'] ),
		),
	),
	'orderby'   => 'title',
	'order'     => 'DESC',
	'post_type' => 'attendee',
    );
	
	// Get attendees 'posts'.
	$posts = get_posts( $attendees_args );
	
	// Loop attendees list and add post title.
	foreach ( $posts as $post ) {

		setup_postdata( $post );

		$name = strtolower( $post->post_title );
		$char = $name[0];

		if ( ! isset( $sorted[ $char ] ) ) {
			$sorted[ $char ] = array();
		}

		$sorted[ $char ][] = $post->post_title; // Or, whatever you want to output.
	}

	// Sort alphabetically by character.
	ksort( $sorted );
	
	// Set attendees list which we return.
	$attendees_list = '';

	// Loop through the sorted character.
	foreach ( $sorted as $character => $attendees ) {
		
		// Get first letter (A, B, C, or D and so on).
		$attendees_list .= '<span class="attendees-first-letter">' . $character . '</span></br>';
		
		// Loop through each post for the character.
		$attendees_list .= '<ul class="attendees-list">';
		foreach ( $attendees as $attendee ) {
			$attendees_list .= '<li>' . $attendee . '</li>';
		}
		$attendees_list .= '</ul>';
	
	}
	
	// Reset post data.
	wp_reset_postdata();
	
	// Return attendee list.
	return $attendees_list;
}
add_shortcode( 'attendees', 'prefix_features_attendees_shortcode' );