How to setup a shared ccache

The easiest solution: create a new group (e.g. "devel"), and make all developers members of it. Give read/write permissions to that group on the directory hierarchy where the cache is maintained. The developers will also need to fix their umask.

See the newly written Sharing a cache section in ccache's manual. In essence, use the same CCACHE_DIR setting, set CCACHE_UMASK appropriately and consider using CCACHE_BASEDIR.