How to email a group of people on Outlook on the Web:

1. Login to https://outlook.office365.com 
2. Select the People view.
3. Click on the Arrow next to New contact.
4. Select New Contact List.
5. Name the Contact List.
6. Add recipients, you can type any email address and hit Enter to add them to the List.
7. Click Save and Close.
8. Once the list is created, switch back to your Inbox view. 
9. Select the New Message icon in the top left. A new message window will appear. Personal Contact lists will populate as you type in the To, CC, BCC fields. Type your message and click Send.

Note: Creating a Contact list on the Mac app (not website) is currently not available on the new Outlook app (but is "Coming
soon"): https://support.microsoft.com/en-us/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439#bkmk_coming-soon 

Note: There is a limit of 500 recipients per list. A possible work around is to use multiple groups. If a more permanent solution is needed, please contact your local IT Pro and request an AD Distribution List.