Hi @Tam Quang
I am afraid you misunderstand the guest definition in Teams client. To be noticed that the guest you said is not the real "guest" but just the anonymous external users.
Guest means that anyone with a business account (that is, an Azure Active Directory account) or consumer email account (with Outlook.com, Gmail.com or others) can participate as a guest in Teams, with access to teams and channel experiences.
For example:
People in my organization and guests' for the 'Who can bypass the lobby?' but neither a user from another tenant nor a guest (who doesn't log into MS Teams) can bypass the lobby.
Is this user from another tenant joined in your organization? If the answer is no, he cannot join Teams meeting.
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