  Number counts of galaxy clusters offer a very promising probe of the Dark
Energy (DE) equation-of-state parameter, $w$. The basic goal is to measure
abundances of these objects as a function of redshift, compare this to a
theoretical prediction, and infer the values of cosmological parameters.
Various teams have proposed such a measurement, including the South Pole
Telescope, the Dark Energy Survey and the Red-Sequence Cluster Survey. The
specific study discussed here detects clusters and smaller galaxy groups in the
three-dimensional distribution of galaxies inferred from a large spectroscopic
redshift survey. This method allows the abundance, $N$, of groups and clusters
to be measured as a function of \emph{velocity dispersion}, as well as of
redshift, permitting a more sensitive test of cosmology.
