You would need then set the parent folder to 0770
, owned by firebird:somegroup
. it-test
and firebird
should be in somegroup
.
Then set the parent folder to be owned by somegroup
and set the setgid
bit on it. This will result in all subfolders and files owned by the parent folder owner group, not their creator group.
Finally upload all files as 0660, folders as 0770. This will result in both users being able to read and modify the files.