I think you're misunderstanding the 2-sided nature of Sia. There are host and there are renters. The host are the ones that offer up disk space for OTHERS to store their data on. Renters pay Siacoin to use some of that space. The host puts up collateral to ensure that they will store the data for the entirety of the contact.
In your situation, you would need to be on the renter side where you store your files on the Sia Network.
From here, you have 1 option right now and 2 known future options.
You can currently build your own integration of Sia and basically create your own front end. This is the developer route which is more or less where the entire project is at the moment.
The first upcoming feature is sharing files to other Sia users. So you could have the people you want to share your files with sign up for Sia and do it that way.
The other upcoming (long term) feature is sharing files with users outside of Sia. This will be the true blast off moment for the technology where you will see Sia start pulling customers from the major players in the cloud storage industry.
I hope this helps explain things for you.
