In V3 installation cluster can be administered by logging into organisation wide Rancher setup. Rancher is integrated with Keycloak for authentication. To provide cluster access to a user perform the following steps as administrator:
Login into organisation wide Keycloak e.g https://iam.xyz.net. It is assumed that you have admin role in Keycloak.
Create a new user.
Make sure a strong password is set for the same under Credentials tab.
On Details tab you should see Update Password flag under Required User Actions. This will prompt a user to change the password during first login. Disable the same only if you are sure you don't want user to change password.
Login to Rancher as administrator, e.g. https://rancher.xyz.net.
Select a cluster for which you would like to enable access to the user.
Add the user as member of the cluster.
Assign a role, e.g Cluster Owner, Cluster Viewer.