The Subvert platform uses a passwordless sign-in system - you log in with a one-time code sent to your email. The member forum runs on different software that requires its own username and password.
These are two separate systems and your sign-in code from the main platform won't work on the forum. When you accept your forum invite, you'll be asked to create a forum-specific username and password. Keep these somewhere safe - you'll need them each time you sign in to the forum.
We know this isn't ideal and we appreciate your patience. It's a current limitation of how the two systems work together.