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Hi Lee,
Welcome to the Microsoft Q&A forum.
Based on my test environment, when I create a new user account and a new Security Group, then add them to the SharePoint site, everything works as expected.
Could you please help me verify the issue by following the steps below?
- Please try creating a new user account and a new Security Group, then add them to the SharePoint site to check whether the issue still occurs.
- Please verify whether the users and Security Groups mentioned in your post can be found in Teams or Outlook. In addition, please try sending an email to those users to confirm that their accounts are functioning properly.
Based on the information you provide, I will be able to gain a better understanding of the issue you are experiencing.
I look forward to your response.
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