I would like to have a directory that has following properties:

• Many users can copy files into it
• These files can be deleted/changed by these users (user A can delete/modify file that was copied into this directory)

it cant be done using normal file permissions (because permissions are retained on copy).

Here is what I found on the net:

• brainstorm idea
• blueprint

Some use cases:

• Sharing music on local machine
• Simple git repository sharing (just make a bare repository writeable to many people) --- i know that there are solutions like gitosis
• Allow many developers to modify test instance of php app without giving them root (i guess they would copy files) --- I'm leading a team of nonprofit junior developers and I need to keep that one simple!

EDIT

AFAIK setting SGID bit is not enugh, it only affects newly created files --- and basic workflow for these use cases ivnolves copying and other operations (which cleave file's gid unchanged)