Blimey, creating a group on OSX is a pain in the butt. Adding users to it even more so.
Don’t use the NetInfo manager gui tool thing… incomprehensible, dangerous IMHO, and the mac help on it is non-existent. addgroup seems to have gone away.
Do use nicl, the Net Information Command Line utility. You’ll need sudo or to be logging as an administrator. All the usual warnings apply, don’t do it if you’re not sure, don’t do it if you’re scared, don’t do it of it’s not your machine, back everythingup, etc, blah, etc.
Create the group
Find out the next gid. If there’s not one higher than 501 use 501, otherwise add one to the greatest number in this list: nidump group . This will also do it and just return the highest currently used number nidump group . | cut -d: -f3 | sort -n | tail -n 1