The Autocomplete feature of Microsoft Outlook keeps a list of recently used email addresses that you’ve typed in the To:, Cc:, and Bcc: fields. Until recently, Outlook retained recently used Contact groups as well as email addresses.
A recent Microsoft Outlook update broke this Auto-Complete feature, indeed there is an active petition to bring this back..
Until the Group Autocomplete feature returns, you can use this work-around: type a portion of the contact group name in the “to” field and then click on the “Check Names” button. Your contact/contact group names will appear.