I'm trying to set docker up on a new system, and when running docker info I get:
docker -v
=> Docker version 18.09.5, build e8ff056

docker info
=> Got permission denied while trying to connect to the Docker daemon
   socket at unix:///var/run/docker.sock: Get
   http://%2Fvar%2Frun%2Fdocker.sock/v1.39/info: dial unix 
   /var/run/docker.sock: connect: permission denied

Following the docs, I've tried:
sudo usermod -a -G docker $USER

Which returns no output. When I then run groups:
groups
=> mark adm cdrom sudo dip plugdev lpadmin sambashare

I can see a docker group exists:
less /etc/group | grep docker
=> docker:x:131:mark

And can see that it owns a socket running where the error message states:
ls -la /var/run/ | grep docker
=> 
drwx------  5 root                root                 120 May 25 14:54 docker
-rw-r--r--  1 root                root                   5 May 25 14:54 docker.pid
srw-rw----  1 root                docker                 0 May 25 14:54 docker.sock

So why can't I add myself to that group with sudo usermod -a -G docker $USER ?