How to modify a shared distribution list in Outlook

In Outlook, switch to "Folder View"

Once there, scroll down in the folder list on the left until you find "Public Folders - " and your primary email address.
Expand the folders and select the "External Contacts" folder as shown in the image below.
Note: Do not try to edit a distribution group from the Outlook Address Book. This is the only method proven to work every time.


Now find the group you wish to edit in the right pane, and double left-click to open the "Contact Group" edit window.
Once in this window you can use the "Members" section of the "Contact Group" ribbon to modify the group.