Access the course by signing in at at
https://proton.oli.cmu.edu/session/new with the existing learner account. Make sure to sign in under Learner / Educator Sign In in the middle of the page, not at the top under "Authoring or your LMS".
OR
From proton.oli.cmu.edu, click the blue Learner/Educator Sign In button in the upper right, then sign in under Learner / Educator Sign In in the middle of the page, not at the top under "Authoring or your LMS".
OR
Click the original invitation link provided by your instructor / school and sign in