It's quite annoying how non-simple OSX is to Linux, so nearly all of the commands I'm used to don't actually work anymore so I have a lot more to learn. One of the basics was the lack of
usermod which I use quite a lot while setting up permissions for things like client FTP, shared folders in Samba and vhosts in Apache2. Seems there is no simple way to do this on Mac, and the command is somewhat long winded, but here it is for anyone looking for a solution that works.
dseditgroup -o edit -u adminusername -p -a username -t user group
Not quite as easy to remember as usermod, but I'm sure after a few attempts it will sink in.
adminusername is the name of your admin account and
username is the name of the account you wish to ammend.
group is the name of the group you wish for the user to go into.