If you are a Project Owner, you are able to input scores on behalf of your Reviewers, but only after they've accepted the invitation to evaluate.
To access your Reviewer's Scorecard, click on Reviewer Scores. From there:
1) Select the user who's scorecard you would like to modify.
2) Click on one of the blanks in their scoring to open the Vendor's scorecard under that Reviewer.
3) When their scorecard pops up, you'll see a warning at the top saying that "you are currently viewing User-x's Scorecard. User-x will be notified via emails of any changes you make". An email will be sent to the Reviewer for each change that you make to their scorecard.
If you are scoring a Questionnaire, you'll need to unlock their questionnaire before you begin submission.
Hovering over the lock icon will yield a similar message warning you that if modifications are made to someone else's card, they will be notified.
From there, you can insert any scores on behalf of your Reviewer.