First, a bit of background to explain what's going on: Files in OS X can have two quite different kinds of permission settings applied to them: POSIX and ACLs.
Files always (well, almost always) have POSIX permissions applied, consisting of an owner, group, and others (with some combination of read, write, and execute for each of those). There is no way to control inheritance of POSIX permissions: new items are always owned by whatever user created them, the group assignment is inherited from the folder they're in, and the access is determined by the umask (which is pretty much always: owner gets full access, group and others read only + execute for folders). So POSIX permissions won't work for what you're trying to do.
Files can also have an access control list (ACL) applied. This is a list of access control entries (ACEs), each of which applies to a user or group, specifies types of access (in great detail), whether they're being allowed or denied, and whether the ACE should also be copied to items created inside the folder. That last bit is the part that makes this useful for you; you need to create an ACE on the folder that specifies the group you want, the types of access you want, and full inheritance.
chmod on OS X can manipulate ACEs with the
-a, etc permissions options. If I understand what you want, you'd use this (with your group name and folder path substituted) to create the ACE:
chmod +a "group:examplegroup allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /path/to/folder
Note: the inheritance is not "live", i.e:
- it doesn't apply to items created before you assigned the ACE
- it doesn't apply to items created somewhere else and then moved into the folder.
You can apply it to existing contents by using
chmod -R +a ...). I don't know of a way (except Apple's server admin tools) to force inheritance to items moved into the folder.