In both directories, 'user-regular1' and 'user-regular2' are member of the 'k8s-users' group, 'user-admin' is member of 'k8s-admins' group.
For Active Direcoty, 'user-bind' is a simple user which is member of the default 'Domain Users' group in Active Directory. Hence, we can use it to authenticate because has read-only access to Active Directory.
The mail attribute is used to create the RBAC rules.