Managing Distribution Lists for O365 (Outlook for Windows).

Click New Email.

Image showing New Email button

Click Address Book.

Image showing message ribbon and address book button

Select the Address Book dropdown and select All Distribution Lists

Image showing Address Book, selecting All Distribution lists for search

Search or browse for the distribution list name, once found right-click and select Properties.

Image showing distribution list right-click menu

Click Modify Members

Imaging showing distribution group properties

Click Add, or select a member and click remove.

Image showing Contact Group Membership

Browse or Search for the user, you want to add to the list. Once found select the user and click the add button. Once you have added all the users you want click ok.  

Image showing the Add Users window