Help Needed; Creating User Portal

Hello, I’d like assistance creating a specific member area. To help with answers I’ll provide clear detail. The type of member area I need would prevent the “sign-up” feature. I’d like to provide specific users access credentials (username/password) which they can later change. I’d like to do this to prevent having to manually screen, and approve users who request membership approval. Second, I’d like to have a page within the member area from which users submit documents (I have built one, and have a database that stores such submissions). Third, and this is the tricky one (at least for me), I’d like to be able to share specific documents with specific users. I know that it is possible to share, and password protect, specific pages and that it is possible for me to share documents via the dropbox app in wix. The issue that I’ve encountered with those two options are that 1. I cannot limit the files shared in the dropbox app for only specific members, and 2. using the password protected page alternative would mean that I’d have to internally create new pages for users every time to simply share documents with specific users – Key Word SPECIFIC. My end goal is to have a custom members area that allows me to: 1. Collect submissions from users and store them in the data base I created, and 2. to be able to share documents with specific users which would be viewed and downloaded from the members member area. I want to be able to share a document with member “A” without members “B” or “C” having access to them as well. Any help would be much appreciated, thank you!

Just in case my explanation was confusing (as I assume it might be): Lets say member “A” submits a document which gets stored in my database. Lets say that I then would like to share that document with member “B,” but ONLY member “B,” and have that document appear in a “shared documents” page (for the sake of this example) in their members area. Hopefully someone with more knowledge can help me solve this. Thanks again!