WordPress custom post type authors system - http://sltaylor.co.uk/blog/custom-post-types-authors-custom-roles/

custom_post_type_authors.php

<?php

add_action( 'cmb2_init', 'pilau_cmb2_custom_fields' );
/**
 * Register custom fields
 */
function pilau_cmb2_custom_fields() {

	// Loop through CPTs
	foreach ( get_post_types( array( '_builtin' => false ), 'objects' ) as $cpt ) {
	
		// Check for support for custom author
		if ( post_type_supports( $cpt->name, 'pilau-author' ) ) {

			$cpt_author_options = array();
			// Gather users that can 'edit_posts' for this CPT
			foreach ( pilau_get_users_by_capability( $cpt->cap->edit_posts ) as $cpt_author ) {
				$cpt_author_options[ $cpt_author->ID ] = $cpt_author->display_name;
			}
			
			// Register box
			$cmb = new_cmb2_box( array(
				'id'			=> 'cpt_author_' . $cpt->name . '_box',
				'title'			=> __( 'Author' ),
				'object_types'		=> array( $cpt->name ),
				'show_on_cb'		=> 'pilau_cmb2_show_on_custom',
				'show_on_custom'	=> array(
					'user_can'		=> 'edit_others_post_types',
				),
				'context'		=> 'side',
				'priority'		=> 'low',
				'show_names'		=> false,
			));
			$cmb->add_field( array(
				'name'				=> __( 'Author' ),
				'id'				=> '_pilau_author',
				'type'				=> 'select',
				'options'			=> $cpt_author_options,
				'default'			=> get_current_user_id(),
				'on_front'			=> false,
			) );

		}
	}

}

/**
 * Get users by capability
 */
function pilau_get_users_by_capability( $cap ) {
	static $all_users = null;
	$users_with_cap = array();

	// Only get all users once per request!
	if ( $all_users === null ) {
		$all_users = get_users();
	}

	// Filter by capability
	foreach( $all_users as $user ) {
		if ( $user->has_cap( $cap ) ) {
			$users_with_cap[] = $user;
		}
	}

	return $users_with_cap;
}

/**
 * Callback to handle custom show_on restrictions
 *
 * @return	bool
 */
function pilau_cmb2_show_on_custom( $cmb ) {
	$show = true;
	$show_on_custom = $cmb->prop( 'show_on_custom' );

	if ( ! empty( $show_on_custom ) ) {

		// Loop through restrictions
		foreach ( $show_on_custom as $show_on_type => $show_on_condition ) {

			switch ( $show_on_type ) {

				case 'user_can': {

					// Special capabilities
					switch ( $show_on_condition ) {
						case 'publish_post_types':
						case 'edit_others_post_types': {
							$screen = get_current_screen();
							$cap_for_posts = str_replace( '_post_types', '_posts', $show_on_condition );
							$post_type_object = get_post_type_object( $screen->post_type );
							$cap = $post_type_object->cap->$cap_for_posts;
							break;
						}
						default: {
							$cap = $show_on_condition;
							break;
						}
					}

					// Check user capability
					$show = current_user_can( $cap );

					break;
				}

				case 'mime_types': {
					// Check for media MIME types
					if ( ! empty( $show_on_condition ) && is_array( $show_on_condition ) && in_array( 'attachment', $cmb->prop( 'object_types' ) ) ) {
						$show = in_array( get_post_mime_type(), $show_on_condition );
					}
					break;
				}

				case 'exclude_ids': {
					// Check for excluding based on IDs
					if ( ! empty( $show_on_condition ) && is_array( $show_on_condition ) ) {
						$show = ! in_array( $cmb->object_id, $show_on_condition );
					}
					break;
				}

				case 'include_ids': {
					// Check for including based on IDs
					if ( ! empty( $show_on_condition ) && is_array( $show_on_condition ) ) {
						$show = in_array( $cmb->object_id, $show_on_condition );
					}
					break;
				}

			}

			// If a condition has failed, break out
			if ( ! $show ) {
				break;
			}

		}

	}

	return $show;
}

add_filter( 'cmb2_override__pilau_author_meta_save', 'pilau_cpt_author_meta_save_override', 0, 4 );
/**
 * Override CPT author meta save in order to store as post author
 */
function pilau_cpt_author_meta_save_override( $override, $data_args, $args, $field ) {

	// Checks to avoid infinite loops
	// @link	https://codex.wordpress.org/Function_Reference/wp_update_post#Caution_-_Infinite_loop
	if ( ! wp_is_post_revision( $data_args['id'] ) ) {

		// Remove filter to avoid loop
		remove_filter( 'cmb2_override__pilau_author_meta_save', 'pilau_cpt_author_meta_save_override', 0 );

		// Update post author
		// Will return non-null value to short-circuit normal meta save
		$override = wp_update_post( array(
			'ID'		=> $data_args['id'],
			'post_author'	=> $data_args['value'],
		));

		// Add filter back
		add_filter( 'cmb2_override__pilau_author_meta_save', 'pilau_cpt_author_meta_save_override', 0 );

	}

	return $override;
}


add_filter( 'cmb2_override__pilau_author_meta_value', 'pilau_cpt_author_meta_value_override', 0, 4 );
/**
 * Override CPT author meta value in order to fetch post author
 */
function pilau_cpt_author_meta_value_override( $data, $object_id, $data_args, $field ) {
	return get_post_field( 'post_author', $object_id );
}