  We develop a method to measure the probability, P(N;M), of finding N galaxies
in a dark-matter halo of mass M from the theoretically determined clustering
properties of dark-matter halos and the observationally measured clustering
properties of galaxies. Knowledge of this function and the distribution of the
dark matter completely specifies all clustering properties of galaxies on
scales larger than the size of dark-matter halos. Furthermore, P(N;M) provides
strong constraints on models of galaxy formation, since it depends upon the
merger history of dark-matter halos and the galaxy-galaxy merger rate within
halos. We show that measurements from a combination of the 2MASS and SDSS or
2dFGRS datasets will allow P(N;M) averaged over halos occupied by bright
galaxies to be accurately measured for N=0-2.
