Thanks for reaching out.
I understand that guest user you are inviting to add in your application is not able to accept the invitation due to the below error :
The reason is the guest you are inviting is already tied to the Microsoft account.
There might be chance user is not able to recall or again want to create Microsoft account with same gmail account then user can proceed to recover its password and create account again by clicking on 'Create and account anyway' by verifying the code sent on his gmail account and then accept the invitation.
Another way is to ask user to provide you with a different email address which is not tied with Microsoft account to avoid that process.
Hope this will answer your query.
Thanks,
Shweta
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