Since we use Mercurial, getting the source requires cloning the Assembla banjo repository:
hg clone http://hg.assembla.com/banjo
You need to request write permission to contribute your changes back to the Banjo project. The mailing list is a good place to do this.
Mercurial makes uploading code easy. As you edit the code, use hg commit to individually check your changes into your copy of the repository. Be sure to provide a good log message so team members know why you made the change. When you are ready for others to use your changes, just hg push and all your changes should go back to the Assembla repo which you originally cloned.
You need to make sure to include the same email address you have registered with Assembla, or Assembla will see your commit as "Anonymous".