Changes made to Encryption Users will propagate the machines as they synchronize with ePO. Generally machines connect once an hour but could be different based upon the McAfee Agent General Policy settings.
To speed this along, from a client machine, open the McAfee Agent Status Monitor and Drive Encryption System status and click Collect and Send Props button. When the MDE status window says, "Created event ...", click the send events button to speed the process along. Once policy enforcement is complete, the machine should have received the new user and instered user into the PBFS.
You can verify that the user has been added in 4 ways:
1. View the MfeEpe.log found in c:\program files\McAfee\Endpoint Encryption Agent\MfeEpe.log
2. View the Machine Info file that can be exported in the McAfee Drive Encryption System Status Monitor
3. Query MfeEpeHost for the user
MfeEpeHost.exe -haveuser <username>
4. Reboot and see if the user is accepted at preboot.