usermod -a -G groupname username
sudo usermod -aG <groups> <username>
sudo groupadd $GROUP
sudo usermod -aG $GROUP $USER
sudo dseditgroup -o edit -t user -a <user> <group> # OSX
usermod -a -G groupname username
sudo usermod -aG <groups> <username>
sudo groupadd $GROUP
sudo usermod -aG $GROUP $USER
sudo dseditgroup -o edit -t user -a <user> <group> # OSX