The distribution group can be found in the Outlook GAL (Global Address List) or the Groups List.
OR
If a non-approved user tries to send an e-mail to the distribution group, a message will be displayed that permission is required to proceed:
How to add/remove distribution list members
Open the distribution list from your Outlook address book – locate and double-click the distribution list POTS users
To add/remove users select Modify Members...
If you wish to ADD a user, click ADD and select the user from the GAL, click ADD. Once all users are added, click OK
If you wish to remove a user, select the user and click Remove