Add a user or contact to an Office 365 distribution list
As the admin of an Office 365 organization, you may need to add one of your users or contacts to a distribution list (see Create distribution lists in Office 365.) For example, you can add employees or external partners or vendors to an email distribution list.
Add a user or contact to a distribution list
Sign in to Office 365 with your work or school account.
Select the app launcher icon and choose Admin.
Can't find the app you're looking for? From the app launcher, select All apps to see an alphabetical list of the apps available to you. From there, you can search for a specific app.
Choose Groups in the left navigation pane.
On the Groups page, select the distribution list you want to add a contact to.
In the Members section, click Edit.
On the View Members page, click or tap Add Members, and select the user or contact you want to add to the distribution list.
Click Save and then Close.
If you haven't created the contact yet, do that first as shown in this video.
Learn how to send email as a distribution list in Office 365.