
Yes, there is Microsoft Licensing Guide but there is fair bit of ambiguity (E.g. How external users are licensed for D365 Customer Service module).
Hey!
What are your exact questions? external users need to be licensed with a Dynamics 365 License (on this case either D365 Customer Service Enterprise or D365 Customer Service Professional, or D365 Customer Engagement) from the tenant where the Environment/App is located. Another possibility is to use a PowerApps per user license, but this doesn't allow using some entities. Similar will happen with D365 Team Member licenses (won't allow you to use all features/apps, only the Customer Service Team member app)
If you have several questions, you can also open a support request to Microsoft with all your doubts.
Regards,