Problem
You want to allow access to files or directories, without giving exclusive use of a given group, user, etc.
Solution
Use setfacl. 🙂 See the example and reference tabs.
Example
setfacl -m user:USERID:rw- filename
getfacl filename
Can also do this: getfacl filename1 | setfacl -f – filename
Performing an ls -ld filename, will show a plus in the permission column.