This one isn't self explanatory, unfortunately... :(

But it's a pretty good inside joke among my friends at work that have read the same strip.

Here's the setup: Dr. Doom has wandered into Reed Richards lab and tried to destroy him. Instead, Doom's head is turned into a big smiley face. Reed can't even be bothered to turn and look at him... Frustrated, Doom stalks off. Spider-Man, in his pink bathroom and bunny slippers, steps out on his porch to get the paper and sees him walking past. He calls out to Dr. Doom, "Hey! Kool-Aid!" And then Dr. Doom responds:

Its use at this point should be clear. When you put somebody, "On the list!"