I found a solution. Not sure if it is the best solution, but it works:

Logged into the container:
docker exec -it emoncms-docker_web_1 /bin/bash

Installed a text editor:
apt update
apt install vim

Edited the settings.ini file:
vi ../emoncms/settings.ini

Added this to the file:
[interface]
enable_multi_user = true

After refreshing the login page I was able to add more users