It depends on what caused the host to shut down. If it had closed down due to a lack of funding or similar issues and the clients had been given a warning that it was about to happen (and enough time to make alternative arrangements), then I wouldn't do anything.
However, if it was a more sudden affair, and it was clear that it was a scam or the owners fault, then I would do my best to help my members. I would do this the best way I could, however I would imagine it would be by either providing them with free temporary hosting whilst they found another host, and offering them money off at another reliable host.