Either with a Team License (I would assume, I don’t have one) or you actually split up the files/workload and combine it for testing/debugging/etc.
You can achieve this by syncing the unity project using a repository.
The way I have achieved it is by storing the Unity Project directory on a git repository hosted on github.com. Then we would store the big files such as Art and Audio files on Dropbox. Keep in mind that you can do this without having to purchase Team License.
The newest version of Unity 4.2 allows scenes changes to be interpreted as textual changes (i.e not just binary). This makes it easy for version control systems such as git to see the changes happened in a scene.
I would’ve appreciated more information such as: How many people are on your team? Do you need to work on the same scene simultaneously? I’ll just coven all the bases I can think of then…
At a project level - to be able to work on the same game simultaneously - you need to share the game between all the developers. For this, you need to use some sort of version control. I’ve tried using e-mail (zipping the project then emailing my partner), Dropbox, GIT and SVN. See Unity’s tips on version control.
Personally I liked SVN the most, because SVN keeps track of all changes made to the project and you can revert the current project to any change. You can also host your own SVN server on a local computer. I think this is possible in GIT too, but I’ve not tried.
GIT I found to be too complicated. With Dropbox you need to handle taking backups manually, as everyone shares the same project. So (note:) when someone screws up, you’ll be royally screwed if you haven’t backed up your project. You can guess why I wasn’t particularly impressed with the e-mail method…
At a scene level - to have multiple persons working on the same scene - you will need some ingenuity. The main problem is that there is no built-in way to only save changed assets in the scene; everyone needs to save the whole scene at once. So if multiple people have made changes, and someone saves the scene, only the changes made by one person will be saved.
This isn’t really a problem if you have one programmer and one level designer/art person: The programmer works on the code, and the art person works on the scenes. If you have multiple level designers, I suggest splitting your game to lots of small scenes that individual developers can work on.
One method I could see working - if you have one level and two level designers- is have three scenes. One scene is the complete level, and the other two are halves of the complete level. Developers work on their own halves; the only allowed operation on the third scene is to copy-paste a scene that is being worked on.
There are some assets in the Asset Store that (say they can) help with this. (Networked Scene Editor, Scene Merge are ones I found with a quick search.)