Short answer: Dropbox (preferably with Dropbox-aware apps).
Longer answer: It depends a lot on what type the documents are, and what apps you use to edit them.
Anything that only has iCloud as a sharing mechanism won't work, without exporting via email or some other means (which would seem to defeat the point). For the foreseeable future, iCloud documents are tied to individual accounts.
Dropbox is probably your best bet - either a shared account, or shared folders between individual accounts. The app itself can open files in whatever application supports them, but then you're dealing with multiple versions again. If at all possible, use apps that have built-in Dropbox support to avoid this.
If you're dealing with plain text, there is a vast selection of Dropbox-enabled text editors out there. Brett Terpstra has a heavily detailed comparison here.
For other formats, it may be a bit more hit and miss. For Office documents, Office2 HD and Documents to Go both advertise Dropbox sync, but there may be more out there.
For PDFs, PDFPen does Dropbox sharing, and is a great app. PDF Expert has similar functionality.
If you narrow down what sort of document support you need, I'm sure others can chime in with suggestions.
iMessage isn't designed to work with just one Apple ID. Instead of having all the iPhones and iPad on one Apple ID, it will be necessary to set up additional Apple IDs for the different users.
When you setup an iOS device and put in your Apple ID, it provisions ALL such services to that Apple ID. Go into Settings on each device (under Messages and FaceTime) and remove the Apple ID. Replace the Apple ID with a new Apple ID unique to each user. Here is a guide on creating new iCloud accounts for this purpose.
Better yet, provision each account completely with its own Apple ID. If a person wants to download an already-purchased app or song from a different account, you can temporarily change accounts in iTunes or App Store.
Best Answer
Depending on how committed you are, you could install a virtual machine on OSX and from there login to your business iTunes account. Furthermore, it's possible to drag just the iMessages application (from the virtualized OSX instance) to your Dock giving the impression of running two different iMessages accounts simultaneously.
I've accomplished similar tasks using VMware for Mac, but Virtual Box may be a good free alternative.