First, they take $N$ pieces of paper, write down a distinct number from 1 to $N$ onto each and put them into a pot. One of them shakes the pot to randomly pick two numbers he'll then memorize. After they're put back into the pot, all of them including him randomly pick a number. He then announces the two "lucky" numbers and that everyone is supposed to add $X$ to their salary except for those with the lucky numbers, who will subtract $X$ instead. Then they all write down the results. That way, the sum can be worked out, but no one knows which one(s) of the others has/have subtracted 1.
Doesn't work if $N=3$.