According to your description, I have the following questions to confirm with you:
What's the way you added the guest? You could refer to this article.
Have you enabled guest permissions in Teams admin center? The methods are as follows.
- Sign into the Microsoft Teams admin center.
- Select Users > Guest access.
- Set Allow guest access in Teams to On.
If the above issues are ruled out, you have added the guest correctly. After you add a guest to a team, it may take a few hours before they have access.
Besides, if the problem persists, you could contact your administrator for guest access problem diagnosis.
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