1. Log into webmail https://outlook.office.com
2. From the Outlook on the Web application, click the gear icon in the top right hand corner of the screen. Under My app settings select Mail.
3. In the menu on the left hand side of the screen, under General select Distribution groups.
4. Select an existing group and click the pencil icon to change its properties.
5. Edit the group as needed in the Distribution Group window: