If I understand your question correctly, you have a third-party forum setup (Invision), for which you’d like Bubble to manage auth (signup/login)?
If so, I don’t have firsthand experience with this. However, I would attempt to connect the auth form to Bubble via a Bubble API endpoint, such that when the Invision signup/login form is submitted, the Bubble API workflow is prompted to sign the user up or log them in. However, this may be tricky if all of the functionality on another platform is dependent upon Bubble’s auth. This would require auth credentials to be present for otherwise restricted actions, so it would be tricky to implement.