Just of the group that owns the file. To view which group/user owns the file:
ls -l filename.txt
This should return something like this:
-rw-r--r-- 1 masterc users 178k May 30 05:22 logo.png
Which shows group 'users' owns that file, and has read priv's.
You can own the file to a different group in a number of ways:
chown user.group (where user is the user to own it to, and group is the group to own it to) so for sales to own it:
chown root.sales filename.txt
Would change it so root owned it, and the group sales had access to it as well, with the permissions you give to the group owner.