Galaxy Groups within 3500 km s$^{-1}$
  A study of the group properties of galaxies in our immediate neighborhood
provides a singular opportunity to observationally constrain the halo mass
function, a fundamental characterization of galaxy formation. Detailed studies
of individual groups have provided the coefficients of scaling relations
between a proxy for the virial radius, velocity dispersion, and mass that
usefully allows groups to be defined over the range $10^{10} - 10^{15}$
$M_\odot$. At a second hierarchical level, associations are defined as regions
around collapsed halos extending to the zero velocity surface at the decoupling
from cosmic expansion. The most remarkable result of the study emerges from the
construction of the halo mass function from the sample. At $\sim10^{12}$
$M_\odot$ there is a jog from the expectation Sheth-Tormen function, such that
halo counts drop by a factor $\sim 3$ in all lower mass bins.
