On the page where you "Choose horse, owner, riders, and trainer for your entry" you can add an additional rider to your entry, just click "add rider" again before clicking "next".
The way the additional rider is displayed depends on the type of entry blank you are building.
For a "qualifying" entry blank or a breed horse entry blank, the additional rider displays as another row on the entry blank.
"Breed Horse View"
For a "misc hunter/jumper/equitation, hunter breeding" entry blank, the additional rider appears as another tab on the entry blank.
Not a problem, there is a "back" button on the bottom of the entry so that you can return to the "choose" page and add them.
Not a problem. If you haven't saved the entry yet you can just click "Create Entry" again on the nav bar and rebuild the entry. Since you've already saved most of the people and the horse, it will go quick.
That's okay, if you haven't submitted the entry to the Competition Organizer yet. You can select the entry on the "My Entries" page and delete it. Then just recreate it again with the correct information. The horse and your existing people are saved. I promise the process will go quick!
Unfortunately, if you've already paid and submitted the entry to the competition organizer, you can no longer make changes to it. But you wouldn't be able to make changes if you'd mailed it or sent it UPS/Federal Express either. Call the Competition Organizer. Hopefully they can work with you to make the needed changes once they've begun processing Electronic Entries. They can change it in the computer just like they would if you'd written something down wrong on a paper entry blank.