I have my Jenkins X setup fresh (for example using jx create cluster gke) and it works great for me.
How do I add another user to work with it (on their own machine)?
Jenkins X uses Kubernetes permissions, so the new user will need to be setup to access the cluster.
This is currently documented here: https://jenkins-x.io/faq/setup/#how-do-i-add-a-user-to-my-jenkins-x-installation