Maybe Greg and Chris were trying something like this:
But obviously with some sort of hallucinogenic.
-They see you (a mark) and your friends at the casino (they're there all the time right? why? To scope out people with cash), they see you staying there for the second time that month (Greg had been there the whole month right?).
The first encounter ANY of you have with any of them is in the high-roller section. To them at the moment you guys look like the perfect crime, young, little bit reckless, like a drink etc.
They make friends with you quickly and start paying for stuff. The amount they are paying for is only a fraction of the amount that they perceive that they can get from you.
They get a hooker to distract one of your friends (I'm guessing he's ether the most physically fit/biggest guys in your group, or somebody they perceive as a threat through his knowledge as they may see him as a bit more switched on)
They then take the 3 of you down to the casino where the shit hits the fan. 3 people are a lot easier to separate than 4 or 2 because 3 is a crowd right? They could separate you from the other 2 for a decent amount of time before your friends realised because they would be busy talking to eachother.
You can hear Greg trying to encourage you to start tripping balls. Why would he do that? If they had dosed you as a joke it would be made 'public' throughout the group for people to laugh. Why would he try to keep something like this between the two of you?
Luckily you have the willpower and presence of mind to fight the substance and remain relatively compos-mentis.
My guess is that they were trying to rob you by dosing you with something, then taking you to an ATM or whatever and emptying your account. The money you spend in a casino is supposed to be disposable funds, so if they see you dropping $100 bills they will wonder what else you've got.
If you really wanted to go down the conspiracy route you could also wonder whether or not it was just your wallet they were after and not a kidney or a lung.