Alright, I was able to solve it. I changed my overall account's email address to be the same as the email address that received the invitation link. Previously, my personal email was my account email and a "username" email was the company email address. Now, they are both the company address. Once that change was made, following the link on my mobile phone and signing in with this email address worked for being granted access.
This seems like a corner-case that Intuit will need to investigate. I think it probably is doing a check against the primary account address and failing for some. Hope that helps!
