In retrospect

Things I wish I knew / had done... no claim to how correct they are.
1. Set up some guidelines with your friends before the first party (or sooner rather than later), to keep each other in check and make sure no one accidentally strays past their comfort zone.
If your friend / whoever is in an altered state of consciousness and has wandered off to a sketchy place, it doesn't matter wtf other "more experienced" upperclassmen say, or if you haven't set up guidelines with them, or you don't know the place they're at. They can always get their fix at another party--it's better to err on the safe side. Especially if she's the only girl in a roomful of guys, more likely than not something is wrong. Discretely talk to someone--try girls over guys--don't give up until you get someone who'll go and just check up on her with you.

Don't be afraid to watch over your friend / limit their consumption / whatever for fear of looking uncool / lame. More likely than not, they won't remember anything, let alone whether you were lame or not.