Wilbur did invite everyone. It's just that, well, Eret's studious, and Niki's shy, and Schlatt is... inconsistent at the best of times.

So the only person to come to Let's Get High Off Our Own Supply At My Dorm To Celebrate Having A Supply, You're All Invited, is Alex.

"Alex! Come on in, make yourself at home!" Wilbur hands him a vial. "We have way more than enough for more than two people, but hey, just means we'll have more left over. Makes this a better idea, anyway."