To allow configuring rules to automatically add user accounts to the user-defined subclients of an app, enable the auto discovery of user accounts for the app.
From the navigation pane, click Protect > Applications > G Suite.
The G Suite apps page appears.
Click an app.
The app page appears.
In the General section, click Edit.
The Edit Google mail client dialog box appears.
Select the Enable auto discover check box.
Select one of the methods to auto discover the user accounts:
To add the user accounts that match the filters and wildcard expressions that you provide when configuring rules for a subclient, select Match user name by regular expressions.
To add the user accounts that are part of a sub organization created on Google, select Google user group affinity.