You might want to look into the LinuxQuestions Quick and Dirty Guide to Linux File Permissions
Quick answer: You will want to use the SETGID bit, which makes all files created under a specific directory belong to a specific group. So you add a group called "shared" and chown /usr/local/stuff/shared to root:shared and chmod the directory to g+rw. Then you set the GID bit: chmod g+s /usr/local/stuff/shared
. Now all files created in this directory will belong to $USER:shared.
You may also want to consider using the sticky bit.