How are you signing in? Have you attempted to log in with your Cisco Security Account? Its the same username/password as your AMP account.
If none of those options are working please reach out to Cisco TAC or your Cisco Representative to make sure that the Threat Grid account has been provisioned. Please make sure to include the email that should have access and SO/PO# if you open a TAC case.
Dmitri Krull
Technical Marketing Engineer - Endpoint Security
dkrull@cisco.com
SSCP - 743085