By default only IT admins can modify an email distribution list. However, each email distribution list CAN be assigned multiple "owners" by an IT admin, and then the owners can add/remove users as needed from within Outlook. To request Owner access to a given list, send a message to the Help Desk.

To add/remove users to an email distribution list:
1) in Outlook, click on Address Book in the toolbar

2) locate the desired group and double-click on it
3) click on Modify Members to add and remove