First, go to "My Account" in the upper right hand corner. Then find "Portal Options" on the left hand side then open "Flash Portal Buddies". Where it says "Add Another?" type in the Newgrounds Username of the co-author and click "add!". Now you must wait until your co-author goes onto his or her account and approves you as a co-author. After that's done, the username of your co-author will appear on your Portal Buddies page as an Approved Co-Author.
After all that's done, when you go to "Submit Content" again, you should now be given the option to add another person under the Author Credits category. From there you'll be able to choose an author from your list of approved authors as well as specify their contribution and their revenue share.