group id (gid) under Mac's
I rsync'd a directory and noticed that the gid 5100 was not in my Mac's /etc/group file. So I added the group using the normal method (System Preferences -> Users and Groups) and could not find a place to update the gid. I also checked /etc/group file and was at a loss.
Doing a quick man on dseditgroup, with the information from that post, changing a gid on a group can be done via the following command:
sudo dseditgroup -o edit -i 5100 MyNewLocalGroup