I have a SharePoint Online site that I am sharing with people from another company with an O365 Enterprise subscription. I have added several people to the standard Members group on the site. About 25 people have been able to successfully log into the site, but I have one user that gets the error that their account is not authorized on the site. I have verified that they are on the same domain as the others and that their email address that was entered when adding them to the Members group is correct. They are receiving the invitation email to that email address. We have tried having them log in using IE, Edge, Firefox, and Chrome, as well as using an In Private session to make sure there was not a cached credential issue. I'm not sure what to try next.