be strong. tell her no, and stick with it. tell them you're not going to do it, and to stop asking.
Smoke try it decide whether you like it or dont
That event could ruin a LOT of chances and opportunities in life later on, for just doing something that you didn't want to do in the first place.
Your overthinking it tho..given weed had its affects but could be doing worse besides your with your girl just chill she wants to share the experience with you
If you don't want to, I would say no, and ask her to respect that decision.
Oh and a true friend will respect that. (Leaves more for them, anyway! lol)
You can either let your hair down or stick to your chosen lifestyle.