You don't have group write, so the group can't write.

Also see file access control lists (setfact/getfacl). Set up a default group, so that all new files/directories are added to the group developers.

Installing aclutils

If using Debian, or derivatives such as Ubuntu, then install with apt-get install acl. You may also need to activate for your file-system (But maybe not: I am using ext4, I don't seem to have configured it in my mount options, and it is working)