Lots of things to consider here. If you are talking about each user having their own files (i.e., not shared) then you can just set up offline files for them. When they're offline they can work on the files, and when they connect to the network they will replicate/update to the server. If multiple users have access to the same files then you'll have issues with conflicting versions between users.
And of course, the biggest challenge is to be able to sync with the server any time that they go online, rather than any time that they access the corporate network. Basically the files will only sync when you connect to the corporate network, so you will need to employ some sort of VPN solution to make that connection. DirectAccess is essentially an always-on VPN solution (it's more than that, but for the purposes of this discussion that's the role that it plays). You could use some other VPN solution, but the benefit of DirectAccess is that your users do not have to launch a VPN app and make the connection, it is connected automatically.
You may want to look into a third-party service that is designed for collaborative access over the web. I'm not sure if Google Docs or Live Drive or something similar to that will do the trick, but that's the direction that I would go.
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