I'm not so sure. In my opinion Megan was looking to experiment with Rebecca for a few weeks before that night, but neither of those two were involved with the drugs that night.
I went back and studied my videos of the days leading up to orgy night, whenever they did a massage or mud bath Megan was always paying extra close attention to Rebecca, and I think the sexually charged atmosphere that night was the tipping point. Of course the atmosphere was because of the drugs, but like I said Becca and Megan didn't take any