The following instructions show how to add and remove members to an Exchange Distribution Group from within Outlook on the web (aka OWA). For similar instructions for doing this on a desktop Outlook email client, see Distribution Group Member Management on an Outlook Client.
The following paths are followed: Settings > Mail > Options > General > Distribution Groups
Select the Settings icon, then Mail under Your app settings.
Figure 1: screenshot of settings options in Outlook on the web
On the left-hand side, expand the General section and select Distribution Groups.
Figure 2: screenshot of Distribution Groups
Distribution Groups I own shows each distribution group you have manager rights to. You can double-click your desired group to edit or also type in a search request.
Select Membership. All current members of this group will be displayed. Click the + (plus) sign to Add members. You will be taken to a Directory to search for names to add. To Remove members, select the name(s) and then select the - (minus) sign. Be sure to Save your changes for them to take effect.
Figure 3: screenshot of distribution group membership