Open a web browser and navigate to the "Gies Business - Mailing Lists" web interface (https://lists.giesbusiness.illinois.edu) 
Select “Campus Login” (top-right corner)

Log in using your "Campus AD Credentials"

Select the “Search for Lists(s)” or “My Lists” (bottom center) to find the specific mailing list you want to modify

Search for list (if not using "My Lists" Navigation)

Select on the list name to access its management page

On the list page, make a note of the number of subscribers (i.e. Subscribers:601) and select on the "Subscribers" tab (left-hand navigator)

On the Manage list members page, navigate to the "Add Subscribers" field and provide the new subscriber's email address in the “Email address:” field

Select (check) the “Quiet (don’t send welcome email)” checkbox

Select "Add" to complete the process.

Navigate to "Subscriber Table" to verify the new subscriber's email address has been added.

Manually-added user will display "subscribed" instead of the "uofi>bus-..."(i.) 

The number of Subscribers should also have increased (ii.)

i.ii.