Not knowing very much about networking can make solving an issue like this problematic, as there are numerous solutions to the problem, some which will work better than others for you and your team.
Personally, how I would set it up would be to create a server on your domain with a proper network share folder, and then you can have all of your remote client machines connect to the domain server through a VPN tunnel and they can save all of their work on the network share as if they were working locally. You can then backup the shared folder as you normally would. As an added bonus, you could set that server up as a source control box and use it in the same manner.
Since that method requires a fair amount of networking knowledge and setup/maintenance, it may not be the best method for you. You might be more interested in a service solution like Dropbox or Google Drive, but it’d require uploading/downloading all of the necessary files constantly.
If you have the ability to set up an FTP server, you could solve this problem that way, too. Simply FTP the contents of your directory onto the server when you’re done working. Make the FTP server available publicly, or assign credentials to those you wish to have access.