To do what it says on the tin (or at least in the title) run:
sudo dscl -p / -append /Groups/GROUPNAMEHERE users USERNAMESHERE
It was a pain to find how to do this, so I thought I would post it up to make it easier for others. I discovered that www (the Mac apache user) is not in the wheel group so I added it. It conveniently allows me to chgrp stuff for apache to be able to write too as well as my own user.