I used to use a company for called
Egnyte for cloud storage/backups.
They have an app for the Netgear readynas as well as other NAS/Windows/MAC/Android devices which real times syncs any changes to an online copy, the permissions are handled per account and the permissions sync with any devices it is added to also. I don't remember it being hugely expensive per month, but haven't used it for a while i believe you have a basic hosting plan with a certain amount of space and adding new users incurs a cost so we normally did the accounts by groups.
It did work rather well, would access the data locally through the nas for fast transfer then could also access it at home/about through the web.
Can also map a webdav shared drive direct to online, but transfer may be slow if large files.
May not fit the bill, but may be worth a look.