The easiest way to put this is basically, your the rider's slave lol. (In a good way though)
A few summers ago that's all I did. It was usually just one horse per groom though, $20 for the day. It was alright, nothing special though.
I usually got to the barn before the day before to help the rider bathe, and braid their horse. Sometimes I'd even help clean tack. Then the next day before the show I'd got to the barn before the rider, brush their horse down, removed shavings from mane and tail, cleaned hooves, prepare it for trailer, etc.
At the show I would always carry 2 cloths to wipe foam off mouth, and the other for sweat around the butt, and neck/shoulders. I would also carry a halter/lead (if the bridle was on the horse obviously), a brush for legs, and if the rider had a water bottle I usually had that too.
Then at the end of the day we'd load the horses and head back to the barn to wrap the horses legs, and put them away safely for the night
I know it seems like a crap load since I wrote a paragraph lol. But I had fun doing, I liked being at the shows, and getting involved. And I think it was a good experience too.
Hope that helps! LOL