To discover mailboxes automatically, add the AD group to the Exchange Online app.
After you enable automatic discovery on the app, when a backup operation runs for one of the mailboxes, users groups are automatically discovered and included in the backup.
From the navigation pane, go to Protect > Applications > Office 365.
The Office 365 apps page appears.
In the Name column, click the app to add the mailboxes to.
The app page appears.
On the Content tab, click Add, and then select Add AD group.
The Add AD group dialog box appears.
From the Exchange plan list, select the plan to use for the mailboxes.
From the Select AD groups list, select one or more mailboxes on which you want to enable autodiscovery.
What to Do Next
To automatically discover new user accounts, run a backup operation on the autodiscovery-enabled mailboxes.