I'm thinking Tonido is the best for this right now. I'll tell you why. Because like dropbox you can access your files remotely. It has a backup app plugin, so the syncing would take care there.
You can assign users to view and be able to download, to enable the sharing. It's pretty simple.
Now, if you had the right host, like a VPS, maybe you could load tonido, so it's an always on dropbox that you own. I dig it. I need to use it more. Unfortunately from work, I can't access my home box even with tonido because my work's network blocks acess to the port that tonido uses, like port 10000 something... you can look it up. But there's an iphone, android app.
I could say more. I need to blog about this probably just to reach out to a few others. I've purchased the pro plugins for it, because they were good, but if I had a tonido plug it would come with those.