This applies when setting up a distribution list in MS Exchange for use by any cell phone, not just Blackberry.
Basically, you just setup a Distribution list in Exchange, give it an email address, assign who is allowed to send mail to it and then assign the members from your users or Contacts which you setup in Active Directory. If you setup contacts in AD, don’t forget to “hide” them from the address book or else they’ll be seen by all your users with access to the address book. Setup a new contact on your Blackberry or cell phone with the email address of the distribution list and you’re ready to start sending email.
Don’t forget to test it out before you go live.