Edit Distribution Lists in Microsoft 365

In Outlook on the web, click the settings gear (upper right corner)

Open

In the search box start typing distribution and select Distribution groups (also under General > Distribution Groups)

Open

Click the link to open the distribution list portal

Select ‘Groups I own’ and Double click the group you would like to edit.

Select ‘Members’ then click ‘View all and manage members’

Select ‘Add members’

Search for the user then select the radio button next to their name and click ‘Add’ at the bottom of the flyout