Instructions below

1) Go to Office.com > Outlook icon > Settings (Gear) > General > Distribution Groups > Click on the words this portal

A new tab in your web browser should open and it will take you to the Exchange site

2) Go to the Groups I Own tab > Select your Distribution Group then double click on it to open it > Go to the Members tab

Then click on the + Add members to add members or search for a member to delete from the distribution group/list