The join password is the password of the pmpolicy local account on the primary Privilege Manager policy server. The pmpolicy account password can safely be reset if the password it was set to is unknown. The password is only used by client systems at join time, after which an SSH key is copied over for authentication.
As root or using sudo if your account has the permissions it will look like this for example:
[root@sudo-server ~]# passwd pmpolicy
Changing password for user pmpolicy.
Retype new password:
passwd: all authentication tokens updated successfully.