unionfs/nullfs Help please

[Wojciech Puchar]

what i need:

i have a directory "/home/administration" with files belonging to group 
say "bosses". I've added say "john" and "bill" to group bosses. Now bosses can 
read and write in /home/administration.

Now i want mark, anne and tom to be able to read data from 
/home/administration but not write. Others should not be able to do 
access it at all.

So i created group "administration-read" and added mark,anne and tom to 
it.

Now i wanted using nullfs or unionfs to clone /home/administration to say 
/nullfs/administration-read so it will be read only (no problem) but gid 
of files would be changed to administration-read.

Tried multiple things, to no avail. Seems i don't really understand 
manuals ;)

Any help how to do it this way or other way (but no ACLs please)?