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.
Procedure
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From the navigation pane, click Protect > Applications > G Suite.
The G Suite apps page appears.
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Click an app.
The app page appears.
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In the General section, click Edit.
The Edit Google mail client dialog box appears.
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Select the Enable auto discover check box.
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Select one of the methods to auto discover the user accounts:
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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.
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To add the user accounts that are part of a sub organization created on Google, select Google user group affinity.
 
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Click Save.