Permissions problems

I have an odd problem that has me stumped, but I'm hoping someone out there
has an answer for me...

I setup a Mac OS X server for a small handful of users. From here we have
two primary shares, each with a significant number of sub-folders. On the
server each user has an account defined via Workgroup Manager, as well as a
common group that all users belong to. For data sharing we have two primary
folder defined as shared, giving all users in this group read/write
privileges. Security for this small office is not critical, so all the
files and sub-folders in the two shared folders are setup to be owned by a
user called "Administrator" and the common group, all of which have
read/write permissions. So far so good...

The users are all iBook and iMac users, roughly 75% with OS 9.x and the rest
with OS X. The OS 9.x users connect via the Chooser, use their defined
username/password and have access to the files. No problem.

The OS X clients all log in to their respective machines against their local
user accounts. Each connects to the server via the Go menu and user their
server username/password combinations. Again no problem. However every
once in a while an OS X user opens up a file (90% of which are AppleWorks
files), makes a change and saves the file back at which point the file
permissions get blasted. The file ownership is reassigned to the user who
made the change, which I don't really care about since the primary
permissions are via the common group. What gets weird is sometimes the
group gets changed (or removed!), or the groups permission gets reset to
read only, leaving all other users unable to work with this file until I fix
the permissions.

Ok, so I am not a Unix god, but I do know something about FreeBSD and
OpenBSD, as well as OS X and even (forgive me!) Windows. In none of these
systems have I ever seen a user be able to change the group permissions of
the files like this!

Any ideas on why this happens and better yet how I can stop it from
happening?!

TIA,
Peter