I found it very confusing - we have GH Business, with Copilot active. Could not find a way to upgrade our Copilot to the level required by the agent.
I tried using my personal Copilot for the purpose of trialing the agent - again, a no-go, as my Copilot is "managed" by the organization I'm part of.
Also, you will want to add more control over to who can assign things to Copilot Agent - just having write access to the repository is a poor descriminator, I think.