Cool idea. I'm wondering if there's a way to implement this system without having to rely on trusting the server with your data. Homomorphic encryption seems like it could be useful in this context, since the data is small.
1. First person picks a random number and adds it to their salary. Writes it down ands hands it to the next person. 2. Next person adds their salary to that number and writes it down on a new note, hands to the next person. 3. When the first person gets the note, subtract the original number and divide by number of people.
That method requires that there's no collaboration to remain secure.