The CAS web application is available for demo at https://jasigcas.herokuapp.com/cas
For demo login please visit the documentation default credentials section.
It is important to note that this is a public demo site, used by the project for basic showcases and integration tests. It is not set up for internal demos, and it may go down and up as the project needs. If you have a need for a demo instance with a modified UI, that would be one you set up for your institution.