To add an assistant, go to your Edit Comic area, by clicking on the "EDIT" next to the comic in your user sidebar:
In the Edit Comic area, click on the "assistants" button:
You can use the form on this page to request another user to be your assistant for that comic:
Once you've requested an assistant, a request message will be sent to them which they must accept before they are fully added as an assistant. They can also choose to turn down the request.