umask in gnome 2.4

[Philipp Sutter <sutter ecology at>]

i am using in my network gnome 2.4 on several linux desktop computers.
the home directory of each user is nfs-mounted. 
my problem:
when a user creates a file on a shared directory, then the permissions
of this file are so that other users can't change the file. So, i would
like to change the umask of each user so that every user of a group has
write and read permission on every file in the shared directory. can
anybody tell me where i can do this in gnome?
thanks in advance for any hint.

philipp