Could you please tell me:
Do you want to restrict specific users in your organization to communicate with specific users in the other organization?
I am afraid this is currently not feasible. You can only enable user external access policies to restrict specific users in your organization from communicating with all users in the other organization rather than specific users.
To get this feature more quickly, I have found a feedback in Teams UserVoice and you could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future. Thanks for your understanding and support.
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