Ok, well lets start with the basics before we start trying to Da Vinci any pictures and writing out number sentences.
"Nine groups of kids were partner reading." Partner-reading hints that the kids were reading in groups of 2. We also know that there are 9 of these groups. 9 groups of 2 kids would equal 18 total kids!
Now we can draw this out and write the number sentence as well (using only addition):
2+2+2+2+2+2+2+2+2 = 18
Tip: If your child is a visual learner as opposed to crunching numbers, you may want to start with a quick drawing using stick figures instead of the number sentence.