You can use Azure affinity groups to autodiscover user accounts.
Before You Begin
Enable autodiscovery of user accounts, and then select Azure AD groups. For more information, see Enabling Autodiscovery of User Accounts.
From the navigation pane, go to Protect > Applications > Office 365.
The Office 365 apps page appears.
Click the OneDrive for Business app.
The app page appears.
On the User groups tab, update an existing user group or create a user group:
To add Azure AD groups to an existing user group, right-click the user group, and then select Manage.
The user group page appears.
To add Azure AD groups to a new user group, in the upper-right corner of the page, click Add user group.
The Add user group page appears.
In the Content section, go to Add > Add group.
The Add new content dialog box appears.
Select the Azure affinity groups that you want to use to autodiscover user accounts, and then click Add.
What to Do Next
Run a backup operation on this user-defined user group to back up all the user accounts that belong to the Azure affinity groups that you selected.