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.

Juergen Schulze

A plugin for wordpress which allows you to send an email to the registered blog users. Users can send personal emails to each other. Power users can email groups of users and even notify group of users of posts.
With ability to schedule mails of the newest post in digest form.
! ! ! S T I L L B E T A ! ! !
All the instructions for installation, the support forums, a FAQ, etc. can be found on the <a href="http://1manfactory.com/emu2" rel="nofollow ugc">plugin home page</a>.
This plugin is available under the GPL license, which means that it’s free. But a “thank you” on my blog is highly apprechiated.
Check out my other <a href="https://wordpress.org/extend/plugins/profile/1manfactory" rel="ugc">WordPress Plugins</a>
<h3>Remove plugin</h3>
<ol>
<li>Deactivate plugin through the ‘Plugins’ menu in WordPress</li>
<li>Delete plugin through the ‘Plugins’ menu in WordPress</li>
</ol>
It’s best to use the build in delete function of wordpress. That way all the stored data will be removed and no orphaned data will stay.
<h3>To do</h3>
More translations. Does someone wants to help?

Emu2 – Email Users 2

Send email to users, manually or on schedule (digest of newest posts).  Users can send emails to each other. Export function included. STILL BETA!!!

Better Author Metabox vs Emu2 – Email Users 2