First, create a Git subfolder inside your Dropbox folder. Then you can share the individual projects inside that folder with whomever you want (or just use it for instant offsite backups).
From inside a Git project:
git clone --bare . ~/Dropbox/Git/gitproject.git
git remote add dropbox ~/Dropbox/Git/gitproject.git
When you're ready to push:
git push dropbox master
Your collaborator would work on the project like so (inside the folder where they want their project to live):
git clone ~/Dropbox/Git/gitproject.git
If they want to have a dropbox remote instead of the default origin:
git remote add dropbox ~/Dropbox/Git/gitproject.git
They would then push the same way:
git push dropbox master
To get the other person's changes, it's the standard deal:
git pull dropbox master