I'm not sure if I'm answering your question, but I'll give it a shot:
I'm assuming you are using Outlook/Exchange as I answer this.
The default filter setting for the BlackBerry redirector (this goes for BES as well) is set to "When none of the above rules apply, Don't forward messages to the handheld". This means that any email message that is not directly sent to the user (meaning any group/distribution lists) will not be sent to the BlackBerry.
To correct this,
Open up the BlackBerry Desktop Manager.
Double click on "Redirector Settings"
Click the Filters tab
at the bottom, you will see "When none of the above rules apply" and two choices underneath that:
Forward messages to the handheld
Don't forward messages to the handheld.
If "Don't" is checked, then that's your problem. Simply click the button next to "Forward" and click Apply.
Now messages sent via a distribution list will be sent to the user's BlackBerry device.
I hope this helps!
- Michael