shooflydesign

This plugin can override the Authors metabox on the post add/edit screen, letting users who can edit other people’s posts, reassign a post to others user on the site.
This is especially useful in situations where you’ve created custom user roles that WordPress doesn’t recognize as Authors. If you want to allow your content editors and administrators to assign new or existing content to anyone, you’ll (hopefully) like this plugin very much indeed.
On WordPress 5.0, the metabox will appear below the main block editor (see screenshot). I will be supporting updates to the Status & Visibility box as soon as possible.
I’m not sure how far back this plugin will work, I’m saying it requires 3.5 or greater — it was originally developed using WordPress 4.1.
(Inspired by <a href="https://core.trac.wordpress.org/ticket/18645" rel="nofollow ugc">this trac ticket</a>)

Better Author Metabox

Lets you safely override the built-in Author metabox with one that includes more users.

Applelo

This plugin improves the admin users search. Just configure what datas you need to search (or not) and that all 🙂 
You can choose to configure what default wordpress search (login, url, email, nicename, display_name) and extends search with user metas (last_name, first_name…).
Free tip: this plugin do more sense with <a href="https://wordpress.org/plugins/codepress-admin-columns/" rel="ugc">Admin Columns</a> 😉

Better Author Metabox vs Better Admin Users Search