We had an employee setup in our HipChat Self Hosted Server and we did not have the Guest Console admin password.
We tried the default and that did not work. We have scoured Google and have yet to find any documentation Atlassian or anywhere else.
What do you mean by the guest console?
If you're talking about the server administrator's password, you can reset that by SSHing into the server, and then run the following command:
hipchat service --reset-owner=EMAIL
If you're talking about access to the admin console itself, you could get access by injecting your public SSH key into /home/admin/.ssh/authorized_keys. On AWS this can be done via a user-data script, or by detaching the EBS volume, updating the SSH key, and attaching it to the instance again. On VSphere this can be done through the vifs command.