Updating Distribution Lists in Outlook Online
Log into Outlook online (https://outlook.office365.com/mail/inbox).
Click on the gear icon in the top right.
Click “View all Outlook settings” in the bottom right.
Click General in the top left of the box that pops up if it’s not already selected.
Click Distribution groups.
Wait for the group options to load.
Select the group you wish to modify membership of in the right list, Test Group in this example.
Click on the pencil icon to edit.
Click membership in the box that pops up.
Click the + icon to add people to the group or select the person you wish to remove and click on the – icon to remove them.