This requires customization.

When uploading a file, the default sharing permissions match the Folder it is added to.

Using Developer Mode, you can create an Automation Rule that is triggered when a new Document is created to email all Users the Folder is already shared with.

Steps for Odoo 19.0:

First, Uncheck the Restrict Template Rendering option:

Next, create an Email Template you will use to notify people a new Document is available.

You can toggle to "code view" by entering text, selecting it, then expanding the toolbar that pops up:

Once you are in Code View, enter the following for the Body:

Copy/Paste version:

<div>
Click
  <a t-att-href="object.folder_id.access_url">
    HERE
  </a>
to access it now!
</div>

Settings for the Template:

Copy/Paste version:

{{ object.folder_id.access_ids.mapped('partner_id').ids }}

Next, create an Automation Rule like this, using the Email Template you just created:

Next, allow this Automated Action to run on each Folder you are sharing:

Result: Email before it leaves Odoo:

Email as received: