Build Functionality that Notifies Users via Email when New Document is Uploaded

Looking for a freelancer to develop a piece of functionality on a WordPress website - specifically a private member portal we have built on Wordpress. Our client is interested in allowing members (indexed as WP Users) to opt-in to receive an email every time a new document is uploaded within their user group. Documents would be uploaded by the site admin. We use Wordpress and Gravity Forms, so looking for a custom plugin / solution that would work with our framework. Timing is flexible on implementing this piece of functionality, but would want to have it built & added within the next few months. If you know of an out-of-the-box solution, we're happy to consider it as well and hire you to implement it (doesn't necessarily need to be custom software). I've added in an estimated budget, but we will consider bids that are higher or lower, depending on the solution that you propose. When you are proposing a solution, please give us a brief outline of how you would build this functionality and whether your solution uses any off-the-shelf products (and if so, what the costs of those are). Thank you!