Separate names with a comma.
a show that was submitted already
No you can't unfortunately. Once it's submitted, it's final.
I made a mistake once and forgot to add one person's order, which was small. I had no other shows coming up (if you have one, ask the host if you can add it to hers for the shipping deal) so I ate the cost of the rest of the shipping and had it still sent to the host (I was new and didn't want anyone to know I goofed).