  We investigate the incidence of major mergers creating >10e11 Msun galaxies
in present-day groups and clusters more massive than 2.5e13 Msun. We identify
38 pairs of massive galaxies with mutual tidal interaction signatures selected
from >5000 galaxies with >5e10 Msun that reside in 845 such groups. We fit the
images of each galaxy pair as the line-of-sight projection of symmetric models
and identify mergers by the presence of residual asymmetries around each
progenitor, such as off-center isophotes, broad tidal tails, and dynamical
friction wakes. At the resolution and sensitivity of the SDSS, such mergers are
found in 16% of high-mass, galaxy-galaxy pairs with magnitude differences of
<1.5 and <30 kpc projected separations. We find that 90% of these mergers have
nearly equal-mass progenitors with red-sequence colors and
centrally-concentrated morphologies, the hallmarks of dissipationless merger
simulations. Mergers at group centers are more common than between 2
satellites, but both are morphologically indistinguishable and we tentatively
conclude that the latter are likely located at the dynamical centers of
recently accreted subhalos. The frequency of central and satellite merging
diminishes with group mass consistent with dynamical friction expectations.
Based on reasonable assumptions, the centers of these massive halos are growing
in stellar mass by 1-9% per Gyr, on average. Compared to all LRG-LRG mergers,
we find a 2-9 times higher rate for their merging when restricted to these
dense environments. Our results imply that the massive end of the galaxy
population continues to evolve hierarchically at a measurable level, and that
the centers of massive groups are the preferred environment for merger-driven
galaxy assembly. (abridged)
