Create a bare repository
Create the directory which you want to share:
Initialize the repository and modify the Git config to enable shared mode (not needed for Dropbox but good to have if you move your bare repository to a proper server).
git init --bare
add sharedrepository=true to the config file.
Now you have a proper bare repository.
Share the repository in Dropbox
Copy the folder you initialized in your Dropbox folder. If you like, you can share it with your fellow developer(s).
Connect to your "Central Repository" and share your work
You can connect to your bare repository by executing the following command:
git clone PATH_TO_DROPBOX_FOLDER/mydev.git
Create some work to share
git add my_example_file.txt
git commit -m "my first commit"
Git commit commits your work to the local repository but doesn't commit it to the central repository, so to share your work you need to push your work to the central repository.
To get the work from the central repository, you need to do a pull operation:
Last I should warn you that we've had some issues with corrupting the central repository. We got everything working again by doing a new commit and push by the person who last changed the repository. However, despite the occasional corruptions in the central repository, there are several advantages to using Dropbox as the central repository, and it will make the development work a lot easier.