Let's see....These are going to be more Musicians than anything...
Weed - David Cross, Brendan Small (adult swim show creator), and Weird Al, Isaac Brock, J.K. Rowling.
Alcohol - Isaac Brock and the rest of Modest Mouse (with instruments in hand), Trent Reznor, Chris Rock
MDMA - Edward Norton or Johnny Depp, then invite my girlfriend. Yeah, I'd do that for her, especially if all three (or 4) of us were high together. Or maybe instead of good-looking males for her, maybe Christina Ricci and/or Keira Knightley for me %) . Or Isaac Brock just for talking/music making purposes.
Speed - Isaac Brock, Marilyn Manson back in his crazier days, Trent Reznor (get down on some crazy music programming).
Shrooms - The puppets from Sesame Street, while on the set talking about numbers and such. That'd be interesting.
Salvia - J.K. Rowling, and once again, Isaac Brock.