<?php
$user = get_field('staff_user_id');
print_r($user);//中身を確認
?>
そうすると
これが入ってました。
Array ( [ID] =>
[user_firstname] =>
[user_lastname] =>
[nickname] =>
[user_nicename] =>
[display_name] =>
[user_email] =>
[user_url] =>
[user_registered] =>
[user_description] =>
[user_avatar] => )
echo $user['ID'];
で出力出来るかと思います。