WordPress.org

Ella

This plugin allows you to create documents with WordPress, and to share them with others so they can also read and edit. Share the unique link to let anyone edit, or restrict access to specific email addresses.
With the Gutenberg plugin, Docs supports real-time collaborative editing — multiple users can edit the same document simultaneously, seeing each other’s cursors and changes live.
<h3>Sharing</h3>
The share panel (in the document sidebar) lets you control access:
<ul>
<li>Anyone with the link — anonymous users get a randomly generated animal name and emoji avatar (e.g. “Anonymous Fox”). They can edit text but cannot upload files.</li>
<li>Specific people — invite collaborators by email. They receive a magic link to access the editor. Email-invited users can upload files.</li>
<li>Existing users — add collaborators from the user autocomplete. They get full editing and upload access.</li>
<li>Restricted — only the document author and invited people can access.</li>
</ul>
<h3>How it works</h3>
Documents live entirely in wp-admin — there’s no public frontend. Each doc gets a unique and secret shareable URL that opens the editor directly.
Anonymous visitors don’t create any database entries. Their identity exists only in a browser cookie and disappears when it expires.
Email-invited people are created as real WordPress users (without a role) so their edits show up in revision history. They log in via magic links sent to their email.

Docs

how it works? 
Can I use it in the classic editor?

how it works

Create and share documents with WordPress!

9hdigital

This plugin will allow users to chat with the website’s business in real-time by using Facebook Messenger. Users can start a conversation on the website and continue it on other devices at any time.
Please note, Facebook is still currently developing their Customer Chat and so it is still in beta.

WP Chat

Chat with business customers on your website using Facebook Messenger

Docs vs WP Chat