You’ll need to create a user account within Active Directory, then create an Exchange mailbox for the user (you can probably do both at the same time from within the Exchange Management Console). You can then add the user to the Blackberry Server. How you do this will depend on what version of the blackberry server you are using.
After you add the user to the BlackBerry server you’ll assign then an activation password. They will use this along with their email address to complete the Enterprise Activation from their cell phone.