How do I add credits to a video?
First, let's clear up some terminology: Participant (actor, producer, writer...) is the same thing as Channel. If somebody wants to be included in the credits they must have a Channel. Anyone who has uploaded content to Funny Or Die will have a channel. For more info on users and channels, go here.
To add participants/channels to your video, go to your channel's Media page. Where is dat? Then click on the title of the video you wish to add participants to. On the next screen click on the gear in the top right corner of the Participants section. Click on 'Select a channel...' then enter either the channel name or URL. It will do a quick search and then display likely matches.
Pick the Channel you would like to add, enter their role, select editing permissions, and then click 'Add participant'. Keep in mind you can edit or remove participants at any time by clicking 'Edit' next to their channel name.
After you are done an email will be sent to the channel owner:
They will need to click on the link and be logged into Funny Or Die to confirm.