Example of our halo mass estimation model using the group richness, built from mock groups/clusters from the Millennium simulation. The gray lines show the richness of an ensemble of mock groups selected at the redshift of our detected group at z _grp = 0.1867 after incorporating the incompleteness and magnitude threshold of our data. The curve is the spline fit to this ensemble of mock groups, which is then used to estimate the halo masses as a function of the group richness.