Hi @tommy soo
You may only add external access participants to existing or ongoing chats that already include at least one external access participant. Otherwise, you’ll create a new group chat with the internal participants and the one or more external participants that you’re adding.
This is the design of Teams. If you want to invite external users to the group chat and keep the chat history, please make sure that the group chat already has an external user. You can also invite the external user to the group chat as a guest user.
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