$cmb2->add_field( array(
	'name'  => 'Author',,
	'id'    => 'author',
	'desc'  => 'Type the name of the author and select from the options',
	'type'  => 'user_select_text'
	'options' => array(
		'user_roles' => array( 'role1', 'role2' ), // Specify which roles to query for.
	),
) );

array(
	'id'   => 1
	'name' => 'Joe Blogs'
)

/**
 * Takes the id from the database and returns an array for user_select_text
 * @param int $value
 * @return array('name' => string, 'id' => int)
 */
function id_to_user_select_text($value) {
	$user = get_user_by('id', (int)$value);
	return array(
		'name' => $user->display_name,
		'id'   => $user->ID,
	);
}

/**
 * Takes the array from user_select_text and returns the id for the database
 * @param array('name' => string, 'id' => int) $value
 * @return int
 */
function user_select_text_to_id($value) {
	return $value['id'];
}

$cmb2->add_field( array(
	// ...snip...
	'escape_cb'       => 'user_select_text_to_id',
	'sanitization_cb' => 'id_to_user_select_text',
) );