This is my personal opinion, taking into consideration that you need an easy to learn solution and that you are working with documentation notebooks and not plain code (
.m files), so it's more difficult to effectively diff them.
Using version control may be beneficial for your project. The main reason is that it will make it easy to keep track of who changed exactly what.
However, I do not recommend that you use Workbench for this because the tools available for it (eclipse plugins) will not be friendly to Mathematica notebooks. They will not make it possible for you to compare two Mathematica notebook and see what has changed in a human readable way, so you would lose the main benefit.
There are tools that make it possible to diff two Mathematica notebook graphically: the old
AuthorTools` package and the
NotebookTools`NotebookDiff function which is included by default (no packages needed) and seems to do exactly the same as AuthorTools. To use these tools manually is too much trouble, and to set them up to work with a version control system is again not user friendly. So I recommend that you use version control tools which already support Mathematica's diffing tools out of the box.
Such tools do exist: TortoiseSVN for Windows knows how to diff Mathematica notebooks out of the box---no setup needed. ToirtoiseHg has borrowed the same code from TortoiseSVN. The Git tools are generally less user friendly on Windows and as far as I know they didn't borrow the Mathematica diffing tools from TortoiseSVN. Of SVN and Hg I suggest Hg because there are free and popular services that give you Hg hosting, so you can synchronize your repositories. BitBucket is nice and offers public repositories with many users or private repositories with no more than 5 users.
To sum up:
I recommend that you use Mercurial (Hg) and use BitBucket or a similar service to synchronize your repositories.
I do not recommend using the Workbench for version control (there are many plugins, but they're not user friendly enough and they don't support Mathematica specifically). Use TortoiseHg which has support for Mathematica out of the box on Windows (not on other operating systems).
This will make it easy to keep track of changes, see what has been changed precisely and who has changed it, or to go back to previous revisions. However, there will still be a learning curve. I think this will be manageable for your collaborators.
Using any kind of version control will require people to learn a bit about it. Perhaps it's simpler to just use a shared Dropbox folder. This will make sharing the documents much easier (no need for email any more), it's very user friendly, and you have the option to revert to previous versions (for up to 30 days, I think). Disadvantage: you can't see at a glance what exactly was changed; it'll only tell you which file was changed, but not which part of the file.
Personally I use Dropbox when my collaborators are not familiar with version control. For short term projects where you don't really need long term history it's more productive.