  This letter presents results of new high resolution $\Omega=1$ Cold + Hot
Dark Matter (CHDM) and Cold Dark Matter (CDM) simulations. Properties of groups
in these simulations reflect the lower small-scale velocities and the greater
tendency to form distinct filaments on both small and large scales in CHDM as
compared to CDM. The fraction of galaxies in groups and the median group rms
velocity are found to be powerful discriminators between models. We combine
these two features into a very robust statistic, median group rms velocity
$v_{\rm gr}(f_{\rm gr})$ as a function of the fraction $f_{\rm gr}$ of galaxies
in groups. Using this statistic, we compare ``observed'' simulations to CfA
data in redshift space in a careful and consistent way. We find that CHDM
remains a promising model, with for example $v_{\rmgr}(0.45) \approx 125 \pm 25
\kms$ in agreement with the CfA data, while CDM with bias b=1.0
(COBE-compatible) or b=1.5, both giving $v_{\rm gr}(0.45) \approx 400 \pm 25
\kms$, can be virtually ruled out. Using median $M/L$, the observed value of
$\Omega$ is $0.10$ (CHDM) to $0.38$ (CDM).
