The Unix file permission model has provisions for this sort of situation, but unfortunately it's far from perfect and not very flexible. As has been suggested, the solution involves making the files group writable. The unfortunate part is that I know of no consistent, automatic way of doing this in OS X.
In the Terminal, you can use the umask command to set default permissions for any new files you create. The default value is 022, which means that the user gets all permissions, but group and others don't get write permission. You want to give group write permission on all new files, so setting the umask to 002 removes the write restriction for the group.
Problem is, this only seems to work in the Terminal in OS X. Indeed, if you type
umask 002 in the Terminal and then create a new file, it is group writable. You can even put this command in your shell init scripts (.tcshrc or .bashrc depending on the shell) and it will apply to every Terminal session you use.
But most of us work in the gui, and I haven't found any way to set the umask and get it to stick for normal gui processes (Finder or any normal application). Plus, by itself, the umask above means that files are writable by your main group, which is probably staff. Depending on who else has access to the machine, this may or may not be desirable. Unix has a way of telling any folder that all new files created there will have a certain group other than the default, but amazingly it cannot be more flexible and also specify things like the default file permissions on a folder-by-folder basis. Clearly the permission model needs an update, and hopefully Apple is working on something for 10.4? If not, let's go spam them with feedback about this.
Anyway, sorry for the longwinded post. The only real "solution" I can think of is a bit of a kludge but it should work fairly well. The Unix "cron" program allows you to schedule certain commands to run at certain times of the day. You could schedule a job to run every minute and change the permissions of the files in your directory to what you want. That way, any new files created there will have the correct permissions within one minute or less, without your intervention.
To do this, create a new text file with contents like the following:
* * * * * /usr/bin/chgrp admin /Users/Shared/somefolder/* ; /bin/chmod g+w /Users/Shared/somefolder/*
It should all be on one line. Change the path to wherever your shared folder is. Save the file and then open Terminal. From the prompt, type
crontab filename where filename is the name of the file you saved.
Now, this is a total hack. It does a lot of needless work but if you don't have too many files you shouldn't notice any slowdown (I tested with 10000 and it took about 3 seconds on my 733 G4, 100 or so shouldn't be even a little blip). If you have files in subfolders too, then the above commands get a little trickier...
