I'm a client of our accountant who sent me an Intuit Link email. When I click on it, it asks me to either sign up for an Intuit account or to log in. I already have an account that was signed up with Mint.com, and so I provide that credential.
Once logged in, the page still tells me:
You've found Intuit Link.
As a taxpayer, you’ll be able to easily gather and share your tax documents with your accountant. But first, you’ll need to ask your accountant to invite you.If you’ve already been invited by your accountant, then sign out and sign in with the email address your accountant used to invite you.