  This paper presents a study of the specific merger rate as a function of
group membership, local environment, and redshift in a very large, $500h^{-1}
Mpc$, cosmological N-body simulation, the \textit{Millennium Simulation}. The
goal is to provide environmental diagnostics of major merger populations in
order to test simulations against observations and provide further constraints
on major merger driven galaxy evolution scenarios. A halo sample is defined
using the maximum circular velocity, which is both well defined for subhalos
and closely correlated with galaxy luminosity. Subhalos, including the
precursors of major mergers, are severely tidally stripped. Major mergers
between subhalos are therefore extremely rare. Tidal stripping also suppresses
dynamical friction, resulting in long major merger time scales when the more
massive halo does not host other subhalos. In contrast, when other subhalos are
present major merger time scales are several times shorter. This enhancement is
likely due to inelastic unbound collisions between subhalos. Following these
results, we predict that major mergers in group environments are dominated by
mergers involving the central galaxy, that the specific merger rate is
suppressed in groups, and that the frequency of fainter companions is enhanced
for mergers and their remnants. We also observe an `assembly bias' in the major
merger rate in that mergers of galaxy-like halos are slightly suppressed in
overdense environments while mergers of group-like halos are slightly enhanced.
A dynamical explanation for this trend is advanced which calls on both tidal
effects and interactions between bound halos beyond the virial radii of locally
dynamically dominant halos.
