Emily, here is the article that describes how to set it up and what permissions you need.
https://help.mypurecloud.com/articles/send-outbound-email-behalf-queue/
We released this functionality earlier in the year, and we do not automatically change roles for existing customers to give them access to new features - we allow you to enable new functionality for agents or supervisors at your own pace. This article describes how you can add new permissions to a role and at the very bottom, it shows how you can reset any default role - that way you get all the new functionality with one click.
https://help.mypurecloud.com/articles/add-roles/