A couple things
1. Using a VIP in Clearpass
2. Are you promoting the subscriber to publisher?
a. If you are take a look at the logs and see if the promotion is completed.
b. If it does take over as publisher and you still can't add guest please open a TAC case.
3. If you are not promoting the subscriber
a. Then CPPM is working as expected.
b. The publisher is the one that contains the guest user database.
So if the publisher goes down the subscriber will take over for all the auths and accounting for radius and tacacs, but you will not be able to add new guest accounts until the publisher comes back online.
You will need to make sure you not only are you using the VIP but you must also turn on auto promotion in the cluster wide settings.