1 - Open Outlook Web App (OWA) - https://outlook.office.com/

2 - Go to OWA Settings (gear wheel on top right corner) and click in View all Outlook settings.

3 - Click on General button, then in distribution groups. 

In column Distribution Groups I own, double click on Distribution the List that you want to Edit.

4 - A new Window will open with the selected Distribution List.

Click on membership. To add new Members click on Plus Sign.

4.1 - A new Window will open, Click on Default Global Address List and Type on Search Box the name of the User. When located Click on Plus Sign.

4.2 - To remove a member, Locate de the member in the list (1), select-it (2) and click on the minus button (3).

5 - To Add or Remove Owners, click on ownership.

To add follow Owners follow the step 4.1

To remove Owners follow the step 4.2