Group Join Mail sent out to soon? Access denied in SharePoint

Yes, I've been experiencing this behavior frequently, at least with external users. They click the "Read Group Files" in the welcome email and are no granted access to the SharePoint document library. I've also had some external users be unable to access the SharePoint team site after previously having access. Obviously, that's a jarring user experience. I have an open support request with Microsoft on these issues and will try to follow-up whenever I get a resolution. So far, rather than resolve problems I've only stumbled into more issues. For example, I just noticed that this problematic Group can't be managed within Outlook; I'm able to manage others group I've created within the program.