Web Dev : Data storage in cloud

I'm developing a website (for the first time). The users should be able to upload and retrieve files (with restricted access).

Example, One user uploads a file and has access to it. Another user needs access to it. And then only the two have access to the file. I need it all automated, so that when a user asks for access, they get some URL to view the file.

I believe I will require subscription to Amazon S3 or google cloud services (if there is a free alternative where I can test, that would be great). Do these services allow different anonymous users to upload data? Can they do it fairly easily, or would they need basic knowledge of how buckets and objects work?

Can you please tell me how should I proceed?