how to group users into clusters.

code:

I have four users {1 2 3 4} in the system

these four users on grouping can have 15 different number of possibilities.

i made the code in such a way that the users can be grouped under the following possibilities.

possibility=[1     2;
             1     3;
             2     4;
             3     4]

Could anyone advice me how it can be done.