Okay, even though that still had little to no detail I'm gonna tell you that you need to stop worrying. Trust me, if everyone is on the floor grinding and she is the only one at the table, she will feel much more awkward sitting there than trying something new. You also just need to be fun and encourage her. Most girls will not just grab you and start grinding on you out of no-where without you leading. (Unless you were me since I went to 2 proms and that happened to me where even one girl i had a thing with just grinded on me in front of her boyfriend not even on the dance floor, awkward). So just make it seem fun and if worst comes to worst. You didn't grind, big deal it doesn't ruin the prom.
At my schools prom they were completely strict since it is a catholic school and half of the kids just danced in a big sausage fest doing stupid moves and whatever. If this happens at your school do what I did. Tell her you want to dance and give her your hand and if she follows you need to find a spot in the middle of the dance floor and just start. I was the first one to grind but I didn't really care because that got all my friends to come around and do the same. I used to not be as confident but hey, the prom will only be as fun as you make it and some kids said "man our prom sucked" and i thought "well compared to my other one it did but i made this one pretty good" because if i hadn't made my move then I woulda sat there regretting it. So go out and have a good time, make the best of your prom you wont regret it