userquery

// WP_User_Query arguments
$args = array(
	'meta_query'     => array(
		array(
			'key'     => 'exams',
			'value'   => 'exam_id',
		),
	),
);

// The User Query
$user_query = new WP_User_Query( $args );

// The User Loop
if ( ! empty( $user_query->results ) ) {
	foreach ( $user_query->results as $user ) {
		// do something
	}
} else {
	// no users found
}