Today, I was talking to one of my colleague on how a team of developers can work on a BlueMix application collaboratively and we found the concept of Spaces in BlueMix worth using for such a collaboration.
When you login to BlueMix, on the left hand side you will see Space.
If you click on Manage Spaces, it allow you to create your own space.
In any space, you can define users and domains. The user should be an existing user of BlueMix. Once you have the user added, he/she will be able to see any work/application you have on your dashboard under that space. Any changes you make will be reflected to the other users and vice versa.
This way one can collaborate and work on a common application and see each other changes. This also help when one user leave the project and others join. You can keep and adding users for sharing space.