Can't add user to docker group

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 e8ff056docker 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.pidsrw-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 ?