Creating groups and changing group membership in Mac OS X

I want to store some files on a Macintosh (OS 10.2). They should be in the Public folder, but only accessible (= only readable) by certain people. Thus I would like to create a new group that has accession rights to these files. Strangely, the group command doesn't exist on MacOS X (chgrp does exist though). I downloaded the User Manager application. But the version for OS 10.2 was not really easy to use. Thus I used the NetInfo Manager. I duplicated an existing group, changed its name to "gck" and id number and added two users (if you have more than one entry, you have to add them in brackets and comma-seperated). Then I created a directory in my Public folder and changed its group to "gck". Permissions for this directory are drwxr-x---. The files inside this directory also belong all to the "gck" group and have -rwxr----- permissions.