The users are writing to the volume on a PC - is there a CIFS share on the volume, or is it mounted to the PC via NFS?
If they're writing via CIFS, the group the file inherits will be the users' NFS groups, either from the filer's /etc/passwd file, or NIS/LDAP, assuming that the usermapping is properly configured. On the CIFS share, you can set the forcegroup option and/or one of the umask options, to ensure others can write to the files (again, with CIFS access). If you set the umask to 002, files written will be group writeable, which means that any other user who maps to that same unix group will be able to overwrite the file. If you set the forcegroup to whatever group the file gets written with, then everyone who connets will inherit that group, regardless of the group they actually connect to.
If they're writing over an NFS mount to the PC, whatever software is being used to facilitate that should have some sort of user/group/permissions configuration, which you'd have to mess around with.