John Doe does not have a device that can join the group

From the message on the dialogue box and in the absence of a screenshot, I gather you are referring to a third-party application.

If you encounter any challenges with third-party applications, the best course of action is to directly contact the app developer for further assistance. Here's a brief guide on how to do it:

1. Identify the app developer.
2. Visit their website or app store listing.
3. Look for their contact information or support channels.
4. Clearly describe the issue you are facing.
5. Provide any necessary information they may need.
6. Follow their instructions and provide feedback.
7. Be patient and follow up if needed.

By reaching out to the app developer, you increase your chances of receiving accurate and personalized support for the challenges you're experiencing. To contact an App developer --> How to contact an app developer - Apple Support