The auto integration fires after profile update (if Existing selected) or after registration (if New selected). This is no substitute for a mass-import feature, which does not exist. To force users into a group just navigate to User in GJ backend and click "New" then supply the user_id to whatever group you like. With release of CB 2.0 where import/export API is planned to be provided GJ would then receive an import/export feature. However at this time there's no way to mass join groups for users.

Next release will allow you to select whatever trigger you want so you could for instance select the after user synchronize trigger then just sync your users and it'd execute.