Hosting External Hard Drive

If my intention was simply to plug my external harddrive into my rasberry pi, then what would the process be to host that drive as a decentralized server that people could access with a password?
Would it involve having to pay any service to host the server?

So the closest thing that you are describing that FreedomBox has to offer is Syncthing. Any computer that is logged into the Syncthing node that the raspberry pi hosts will then download the data and continuously sync the data on a local connection and on external connections. This is what I use for college. I’ll save a document at home on the local filesystem and then, on campus my documents will be sent to my laptop using my raspberry pi. That system is decentralized and has a form of “account” you can log into.

Another thing FreedomBox provides is Samba. This will allow you to host a network drive. This can be configured to require a password or be open to machines on a local network. I have one of these running on a raspberry pi with an external 1tb hard drive plugged into a USB. I use my openVPN connection to access the drives HOWEVER, I would not do this as you would be giving away your home network to complete strangers. Perhaps you would need to forward the correct ports on a share that’s password protected.

I guess that Pagekite could be used instead.