To change Mac OS X's users and groups from the command line you want to use a command called dscl.

To create a new group (change groupName to your group name - avoid spaces, shortusername should be your short user name):

dscl . -create /Groups/groupNamedscl . -create /Groups/groupName name groupNamedscl . -create /Groups/groupName passwd "*"dscl . -create /Groups/groupName gid 33dscl . -create /Groups/groupName GroupMembership shortusername