Hi @Tanmoy Das ,
Currently we could not add external user in bulk, we could only invite external users to security group one-by-one.
In Azure AD, you could invite external users to join a security group that you have created.
According to the research and test, adding users to groups in bulk is only available for internal users.
More information about Bulk adds group members in Azure Active Directory
Should you have any questions or concerns, please do feel free to contact me.
Yours sincerely,
Renjie Sun
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