  We use the halo occupation model to calibrate galaxy group finders in
magnitude-limited redshift surveys. Since, according to the current scenario of
structure formation, galaxy groups are associated with cold dark matter halos,
we make use of the properties of the halo population in the design of our group
finder. The method starts with an assumed mass-to-light ratio to assign a
tentative mass to each group. This mass is used to estimate the size and
velocity dispersion of the underlying halo that hosts the group, which in turn
is used to determine group membership (in redshift space). This procedure is
repeated until no further changes occur in group memberships. We find that the
final groups selected this way are insensitive to the mass-to-light ratio
assumed. We use mock catalogues, constructed using the conditional luminosity
function (CLF), to test the performance of our group finder in terms of
completeness of true members and contamination by interlopers. Our group finder
is more successful than the conventional Friends-of-Friends group finder in
assigning galaxies in common dark matter halos to a single group. We apply our
group finder to the 2-degree Field Galaxy Redshift Survey and compare the
resulting group properties with model predictions based on the CLF. For the
$\Lambda$CDM `concordance' cosmology we find a clear discrepancy between the
model and data in the sense that the model predicts too many rich groups. In
order to match the observational results we have to either increase the
mass-to-light ratios of rich clusters to a level significantly higher than
current observational estimates, or to assume $\sigma_8 \simeq 0.7$, compared
to the `concordance' value of 0.9.
