Hi @Jasmine Toogood ,
Firstly, please make sure the user accounts who cannot login is the same as the user account in AAD.
Then, please check whether user account can login SharePoint directly(not through Salesforce).
If the account can login without errors, your issue may be caused by Salesforce.
If the account cannot login SharePoint directly, please provide me with specific error message for further help :-)
I hope this information has been useful, please let me know if you still need assistance.
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