Set GUID so the r/w group can share the files created. Then set all others to r/o. ( You also need execute to access the directory)
chmod -r 2775 /directory/in/question
2 - guid -- Files are created based on group
7 -rwx --> Owner
7 -rwx --> Group
5 -r-x --> Others
chown -R owner.group /directory/in/question
Now you should have a working directory for a group to share full access while all others can only read.