Distribution lists (often called mailing lists) can no longer be managed through Outlook. You must now do this by creating a shortcut on your desktop. Please note that this will generally only work on Campus PCs which you have logged into with your IT username (in the format ZYXW321).
On your desktop, right click > New > Shortcut.
Copy and paste the line below in for the location and click Next.
C:\Windows\System32\rundll32.exe dsquery.dll, OpenQueryWindow
Give it a name, click finish.
This shortcut will give you access to a find utility which will let you find and manage the group.
Type in the first few letters of the group name or email address exactly as it displays in Outlook (including spaces and hyphens) then click "Find Now".
Then, in the results pane at the bottom, double-click on the required group to show a window where you can add and remove group members. If the Add button is greyed out then you do not have access to modify the group.
Please note: it can take up to twenty minutes for changes to be reflected in Outlook.