In Microsoft Exchange "Distribution Groups" are used to distribute mail sent to one email address to multiple mailboxes.
If you need to modify the members of a distribution group do the following:
Open up a browser and go to http://cloud.austlink.net
Enter in your administrator credentials
Click on the "Manage distribution lists" link from the menu on the bottom right menu
Click on the "Edit Members" link associated with the distribution group you want to modify
Tick the people you would like to be members of this group
Don't forget to press "Save" at the bottom of the list