Hello Robert ,
You are correct , you require to purchase AIP add-on for all the users. even if you are able to configure the settings/service , you would be out of compliance if a user who does not have this ad-on AIP license tries to use the IRM features within Outlook. Also this may stop working any day if a back-end service level change is done by the product group because as per the documentation , it is expected all the users to be assigned proper AIP licenses for usage of this feature. hence I would suggest to recommend the customer to buy this for all the users.
Hope this clarification helps. In case information in the post helped you , please do mark this as answer so that it can be helpful to other members of the community searching for similar answers.
Thank you.