Make sure that both users are on the access list for the PGP disk. Whoever is on this list the user can use there passphrase that was created to get past bootguard.
To change the username detail, this is done under the Drive encryption section for Detailed Authenication which this will require the Username and password plus Domain but can be saved under policy.
This is about 3/4 down the page in the Drive encryption section of policy.
Simple Authenication is easier because it will only require your password and this will be verifed against the users on the access list for the PGP disk.
If User B is not on the list then the user will not get past Bootguard.