Use the link in your Thatch invite email. Check both your work and personal inboxes for a message from hello@thatch.com with the subject "[Your employer's name] is inviting you to select your health benefits through Thatch."
Your employer may have also given you a Thatch signup link ending in your company's name. That page works too, but only for setting up a brand-new account. Either way, you'll need to use the email address your employer has on file for you.