In Outlook on the web, click the settings gear (upper right corner)
In the search box start typing distribution and select Distribution groups (also under General > Distribution Groups)
Click the link to open the distribution list portal
Double click the group you own and would like to edit. You can also search for a group if you own multiple groups to find the group easier.
You can add/remove users under membership, you can also edit the managers under ownership
Select Default Global Address List, enter user’s name, click + sign next to their name then click Save