Hi there If it was me i would start training from the ground. Maybe when your friends are riding have her next to you with a anti rear bit on attached to her halter. If she goes up you give a tug on the bit. You just have to be careful because you can break there jaw if you use too much force. Another thing you can do when your on the ground is have a dressage whip with you as soon as her legs go to leave the ground you give them a decent tap. When your riding the best thing you can do is keep pushing your horse forward. They need to stop to rear. The less time they have to think about it the better.