You can add evaluators to an existing evaluation by going to Projects > clicking on a specific Project > clicking "Edit Evaluations" button > selecting "Evaluators"
Here's a step by step guide:
1) Go to Projects list page

2) Click on a specific Project

3) Click "Edit Evaluations"

4) "Evaluators" tab.

5) Add people to your list

Let us know if you have any other questions!