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!